KH, GH, and pH trouble

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kymbo

Aquarium Advice Newbie
Joined
May 11, 2025
Messages
7
Location
virginia
My water has always had low ph (according to Petsmart) and I guess that was ok my fish have been great. In October I bought a master test kit because my Oscar stopped eating blood worms and Pet smart said everything was fine. My oscar loves his worms, he comes to top of water and takes them from my fingers. I knew that had to be wrong. What I found was no ph , no kh and gh was ok. High ammonia, high nitrates and nitrates.
I started doing water (tap water) changes like crazy, using Prime. I got advice from a friend to use alkaline buffer and that worked to get everything back in line for a while.
As of today: Ph 7.2 Kh 2 Gh 19 drops, no ammonia no nitrates and 40 ppm nitrates. The ph and kh will change in a day or so. Please any help is appreciated.
 
My water has always had low ph (according to Petsmart) and I guess that was ok my fish have been great. In October I bought a master test kit because my Oscar stopped eating blood worms and Pet smart said everything was fine. My oscar loves his worms, he comes to top of water and takes them from my fingers. I knew that had to be wrong. What I found was no ph , no kh and gh was ok. High ammonia, high nitrates and nitrates.
I started doing water (tap water) changes like crazy, using Prime. I got advice from a friend to use alkaline buffer and that worked to get everything back in line for a while.
As of today: Ph 7.2 Kh 2 Gh 19 drops, no ammonia no nitrates and 40 ppm nitrates. The ph and kh will change in a day or so. Please any help is appreciated.
This is why you should always have your own test kits. ;) Glad you rectified that. (y)
40 ppm nitrate is about the high limit for keeping fish safely so more water changes are your answer for that ( and Oscars in general.) Test your tap water for pH by adding water to a clean cup and letting it sit overnight then test in the morning.
Your higher nitrates are what's causing the pH to fall because nitrate is an acid. With such low carbonate hardness, your nitrifying microbes are probably using that up so there is nothing to keep the pH from falling.
There are a couple of things you can do but the best one(s) will depend on your tap water readings.
On another note, what size tank is your Oscar in? How often do you do water changes and how much do you change each time?
 
My water has always had low ph (according to Petsmart) and I guess that was ok my fish have been great. In October I bought a master test kit because my Oscar stopped eating blood worms and Pet smart said everything was fine. My oscar loves his worms, he comes to top of water and takes them from my fingers. I knew that had to be wrong. What I found was no ph , no kh and gh was ok. High ammonia, high nitrates and nitrates.
I started doing water (tap water) changes like crazy, using Prime. I got advice from a friend to use alkaline buffer and that worked to get everything back in line for a while.
As of today: Ph 7.2 Kh 2 Gh 19 drops, no ammonia no nitrates and 40 ppm nitrates. The ph and kh will change in a day or so. Please any help is appreciated.
This is why you should always have your own test kits. ;) Glad you rectified that. (y)
40 ppm nitrate is about the high limit for keeping fish safely so more water changes are your answer for that ( and Oscars in general.) Test your tap water for pH by adding water to a clean cup and letting it sit overnight then test in the morning.
Your higher nitrates are what's causing the pH to fall because nitrate is an acid. With such low carbonate hardness, your nitrifying microbes are probably using that up so there is nothing to keep the pH from falling.
There are a couple of things you can do but the best one(s) will depend on your tap water readings.
On another note, what size tank is your Oscar in? How often do you do water changes and how much do you change each time?
Hi Andy, thanks for responding. My tank is 55 gallons and I do a 50% water change every other week. I will take a cup of tap water out tonight and test in the morning.
 
Hi Andy, thanks for responding. My tank is 55 gallons and I do a 50% water change every other week. I will take a cup of tap water out tonight and test in the morning.
Just an FYI, a "normal" healthy Oscar should outgrow that tank because it should grow longer than the tank is wide. ( Some of my older Oscars were 15"-17 " long. My Mentor's breeding pair was 18" & 20". )
I'll be checking for your post tomorrow. (y)
 
My oscar is 11" and the placo is 12". They are 4 years does that sound like they are healthy? Thanks again for your help.
 
My oscar is 11" and the placo is 12". They are 4 years does that sound like they are healthy? Thanks again for your help.
And they are in a tank that is only 12" wide so they are really too big for the tank now. One of the reasons a 75 gallon tank is recommended for Oscars is because a standard 75 is 18" wide. My bigger breeder pairs were kept in custom made tanks ( 48" x24" x 20" ) because as breeders, they aren't swimming around much. They stay close to their spawning site. I wouldn't put an adult Oscar, not being bred, in a tank any smaller than 24" wide and 48" - 60" long so they have space to swim around. At 4 years old, your fish is still a toddler. ;) Healthy Oscars can live as long as 15-20 years. (y)
 
Oh wow, I didn't know that, I'll have to figure something out. Thanks.
I tested my town tap water this morning after sitting over night. Here are the results: PH is in between7.2 and 7.6, KH 2, GH 2, zero ammonia, nitrite, and nitrate.
I think the GH reading yesterday may have been wrong, because my water doesn't turn orange at all it just turns a very light green from the start.
Thank you!
 
Hello. I don't believe you can do something for a fish tank that's more important than removing and replacing most of the tank water every few days. This alone with maintain a steady water chemistry and this is considerably more important than a particular water chemistry. If you're not changing out at least half the water every week, you need to work toward this goal. If you can do this religiously, you'll not need to run so many water tests, you'll always know the chemistry is steady. Feeding is nearly as important as the water. Fish have very small stomachs and it doesn't take very much to fill them. Unless, you're breeding fish, you really don't need to feed more than a little every day. The fish will forage and get their exercise. They'll be doing their part to keep the tank clean.

B
 
Oh wow, I didn't know that, I'll have to figure something out. Thanks.
I tested my town tap water this morning after sitting over night. Here are the results: PH is in between7.2 and 7.6, KH 2, GH 2, zero ammonia, nitrite, and nitrate.
I think the GH reading yesterday may have been wrong, because my water doesn't turn orange at all it just turns a very light green from the start.
Thank you!
Okay, so your tap is close to your tank so I suggest you do at least a 30%-40% water change weekly because Oscars are very dirty fish as is a very large Pleco. Your Kh and GH are low so it's near impossible to keep the pH higher without help. My preference is to do more frequent water changes because not only will they raise the pH, they will also help remove any excess waste your filter(s) didn't get, remove nitrates from the water and will replenish the minerals in water the fish absorb out of the water. Other options are to add a bag of crushed coral or shells to your filter which will make your GH and pH higher by melting when the pH gets too low to raise the pH again. The reason I don't like doing this is because it gives the hobbyist a false sense of security that it's all working okay and it does nothing about removing the nitrates which are the main cause of the lower pH. Larger cichlids ( like Oscars) are prone to hole in the head and lateral line diseases as they get larger and the cause is usually a combination of poor water quality and poor diet. Make sure you are feeding the fish a variety of food. No one food is 100% of everything a fish needs to stay healthy so variety is the spice of life. ;) My Oscars ate everything from earthworms to crickets, mealworms, grasshoppers, pieces of fish, pieces of shrimp or whole small frozen prawns, clam meat, fish eggs and feeder fish. So you see, they really can be easily fed a variety of foods as they are basically garbage cans. LOL

Hope this helps. (y)
 
Thanks for responding. I have so many questions but I'll start here. My Oscar has always lived in low Ph because that's what I was told, so I have always kept it no higher than 7, so is that not true?
Also, back in October when I first started having problems I changed the water daily for 30 days then for the next 30 days I did it twice a week it didn't help the nitrates at all.
Every since then I have been doing 50% every other week and just trying to keep the PH stable even though it was high. Could it be the filter? I have a Fluval 306 and it says you don't have change that very often. When I vacuum there is not much in bottom.
My Oscar was eating blood worms but not alot now, and Cich!id sticks. I throw one at a time and it floats down to him. Its funny he floats with his mouth open waiting for me to drop them, just once a day. I have tried shrimp, bug worms, Several other things but he won't eat them. Sometimes I buy feeder fish and he might eat 1 out of 3, most of the time I have to remove them.
Lastly, the alkaline booster was raising the KH but PH was too high I thought. I'm very confused, it sounds like I haven't been taking proper care of my Oscar. Thanks again for helping.
 
Thanks for responding. I have so many questions but I'll start here. My Oscar has always lived in low Ph because that's what I was told, so I have always kept it no higher than 7, so is that not true?
Also, back in October when I first started having problems I changed the water daily for 30 days then for the next 30 days I did it twice a week it didn't help the nitrates at all.
Every since then I have been doing 50% every other week and just trying to keep the PH stable even though it was high. Could it be the filter? I have a Fluval 306 and it says you don't have change that very often. When I vacuum there is not much in bottom.
My Oscar was eating blood worms but not alot now, and Cich!id sticks. I throw one at a time and it floats down to him. Its funny he floats with his mouth open waiting for me to drop them, just once a day. I have tried shrimp, bug worms, Several other things but he won't eat them. Sometimes I buy feeder fish and he might eat 1 out of 3, most of the time I have to remove them.
Lastly, the alkaline booster was raising the KH but PH was too high I thought. I'm very confused, it sounds like I haven't been taking proper care of my Oscar. Thanks again for helping.
All filters need to be cleaned occasionally but with Oscars, because they should be producing more waste than your average fish, that occasionally should be more frequently. What can happen in a filter is the floss or sponge section can get covered causing the water to not flow through them but either around them or only through a smaller open section of the material. The downside of canister filters is that you can't see inside them when they are running. :( I just checked my filters at least weekly or more often depending on what I found inside regarding water flow.
Wild Oscars live in low pH waters in South America but farmed ones have been adapted to much higher and harder waters. Case in point, we have Oscars in Florida's canals that more than likely were hobbyist releases or farm escapees and which have very hard and high alkaline pH ( 8.6-8.8 ) water and are reproducing like crazy. Farmed Oscars are very adaptable.
As for your feeding regimen, it's hard to say if it is enough without seeing a picture of your fish to see body weight and the fullness of it's body structure. What you described with the fish not eating much of other foods definitely sounds like what you want to avoid, the fish getting stuck on only 1 food. Here's the thing tho, Oscars are one of the most intelligent fish species we keep in tanks and because of that, your fish may be manipulating you. ;) That OR, because of the high nitrates, fish suffer organ damage if kept in them too long so that may be what's happening now OR the instability of the water is just making him not feel well :( OR since the fish is severely stunted, ( he should have been 11" at 2 years old and even bigger at 4 years old), it's all catching up with him. There's no real way to know which it is without ruling things out one at a time.
My first action would be to look into a larger tank or pool with more room for him to swim around without bumping into the sides. After that, doing weekly water changes to keep the nitrate level low with checking your filter at least once a week and cleaning as necessary. If none of these get the fish eating more or eating more kinds of food, the last OR is probably the issue and there's not much more you can do. :(
 
All filters need to be cleaned occasionally but with Oscars, because they should be producing more waste than your average fish, that occasionally should be more frequently. What can happen in a filter is the floss or sponge section can get covered causing the water to not flow through them but either around them or only through a smaller open section of the material. The downside of canister filters is that you can't see inside them when they are running. :( I just checked my filters at least weekly or more often depending on what I found inside regarding water flow.
Wild Oscars live in low pH waters in South America but farmed ones have been adapted to much higher and harder waters. Case in point, we have Oscars in Florida's canals that more than likely were hobbyist releases or farm escapees and which have very hard and high alkaline pH ( 8.6-8.8 ) water and are reproducing like crazy. Farmed Oscars are very adaptable.
As for your feeding regimen, it's hard to say if it is enough without seeing a picture of your fish to see body weight and the fullness of it's body structure. What you described with the fish not eating much of other foods definitely sounds like what you want to avoid, the fish getting stuck on only 1 food. Here's the thing tho, Oscars are one of the most intelligent fish species we keep in tanks and because of that, your fish may be manipulating you. ;) That OR, because of the high nitrates, fish suffer organ damage if kept in them too long so that may be what's happening now OR the instability of the water is just making him not feel well :( OR since the fish is severely stunted, ( he should have been 11" at 2 years old and even bigger at 4 years old), it's all catching up with him. There's no real way to know which it is without ruling things out one at a time.
My first action would be to look into a larger tank or pool with more room for him to swim around without bumping into the sides. After that, doing weekly water changes to keep the nitrate level low with checking your filter at least once a week and cleaning as necessary. If none of these get the fish eating more or eating more kinds of food, the last OR is probably the issue and there's not much more you can do.
I don't take great pictures but this my baby. He eats about 10 sticks each night and more if I put them in. I will definitely change the filter tomorrow and keep my eyes on it. So I will do weekly 50% water changes and I guess keep testing? So high PH is ok? Should I add alkalinity booster for Kh? Sorry if I'm repeating myself but I feel very confused.
 

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I don't take great pictures but this my baby. He eats about 10 sticks each night and more if I put them in. I will definitely change the filter tomorrow and keep my eyes on it. So I will do weekly 50% water changes and I guess keep testing? So high PH is ok? Should I add alkalinity booster for Kh? Sorry if I'm repeating myself but I feel very confused.
Based on your picture, you've done a good job keeping him fed. (y) He looks full, I don't see any pitting on his head or lateral line so no issues there. :) It's just the tanks size and his shortness that is in question.
What you might try is a combination of the sticks and mealworms ( since they look like sticks) to see if you can trick him into eating them. Don't feed the sticks one at a time doing this. Toss 3 sticks and a mealworm in at the same time to see if grabs them all. If he does, toss 2 sticks and a mealworm next. If that works, toss 2 mealworms and 1 stick the next time to see if he'll eat them. Once he gets used to the mealworms being like the sticks, he should start eating them alone eventually. You do this with any new food to get them used to understanding that it's all food. BTW, if he doesn't eat the worms, the pleco will most likely eat them so give him overnight to do so.

As for the filter, just remember that the majority of the nitrifying microbe bed is in that filter so you only want to rinse off the sponge or floss so that the surface is rid of any gunk. You don't want to totally wash it out or replace all the material in there. If there is a sponge portion of the filter, just give it a couple of squeezes in some of the water you are removing, to empty it of anything inside the sponge clogging it.

As for the alkalinity booster, you probably won't need it if you are doing weekly water changes. Check your pH before doing the water change and see if it stayed close to the tap water's pH. If it did, once a week water changes should be fine. If it falls below the tap's pH, test daily to see how many days it takes for the pH to fall and do your water changes that many days apart. Changing 25%-30% twice a week is not really different that 50% once a week. FYI, 7.6 pH is not really high. Some African Cichlids come from water with pH in upper 8s. Even marine fish live in pH 8.2-8.4 and THAT isn't really super high either so 7.6 is not really a problem. (y)
 
Hello. I don't believe you can do something for a fish tank that's more important than removing and replacing most of the tank water every few days. This alone with maintain a steady water chemistry and this is considerably more important than a particular water chemistry. If you're not changing out at least half the water every week, you need to work toward this goal. If you can do this religiously, you'll not need to run so many water tests, you'll always know the chemistry is steady. Feeding is nearly as important as the water. Fish have very small stomachs and it doesn't take very much to fill them. Unless, you're breeding fish, you really don't need to feed more than a little every day. The fish will forage and get their exercise. They'll be doing their part to keep the tank clean

Thanks for responding. I have so many questions but I'll start here. My Oscar has always lived in low Ph because that's what I was told, so I have always kept it no higher than 7, so is that not true?
Also, back in October when I first started having problems I changed the water daily for 30 days then for the next 30 days I did it twice a week it didn't help the nitrates at all.
Every since then I have been doing 50% every other week and just trying to keep the PH stable even though it was high. Could it be the filter? I have a Fluval 306 and it says you don't have change that very often. When I vacuum there is not much in bottom.
My Oscar was eating blood worms but not alot now, and Cich!id sticks. I throw one at a time and it floats down to him. Its funny he floats with his mouth open waiting for me to drop them, just once a day. I have tried shrimp, bug worms, Several other things but he won't eat them. Sometimes I buy feeder fish and he might eat 1 out of 3, most of the time I have to remove them.
Lastly, the alkaline booster was raising the KH but PH was too high I thought. I'm very confused, it sounds like I haven't been taking proper care of my Oscar. Thanks again for hhe
Based on your picture, you've done a good job keeping him fed. (y) He looks full, I don't see any pitting on his head or lateral line so no issues there. :) It's just the tanks size and his shortness that is in question.
What you might try is a combination of the sticks and mealworms ( since they look like sticks) to see if you can trick him into eating them. Don't feed the sticks one at a time doing this. Toss 3 sticks and a mealworm in at the same time to see if grabs them all. If he does, toss 2 sticks and a mealworm next. If that works, toss 2 mealworms and 1 stick the next time to see if he'll eat them. Once he gets used to the mealworms being like the sticks, he should start eating them alone eventually. You do this with any new food to get them used to understanding that it's all food. BTW, if he doesn't eat the worms, the pleco will most likely eat them so give him overnight to do so.

As for the filter, just remember that the majority of the nitrifying microbe bed is in that filter so you only want to rinse off the sponge or floss so that the surface is rid of any gunk. You don't want to totally wash it out or replace all the material in there. If there is a sponge portion of the filter, just give it a couple of squeezes in some of the water you are removing, to empty it of anything inside the sponge clogging it.

As for the alkalinity booster, you probably won't need it if you are doing weekly water changes. Check your pH before doing the water change and see if it stayed close to the tap water's pH. If it did, once a week water changes should be fine. If it falls below the tap's pH, test daily to see how many days it takes for the pH to fall and do your water changes that many days apart. Changing 25%-30% twice a week is not really different that 50% once a week. FYI, 7.6 pH is not really high. Some African Cichlids come from water with pH in upper 8s. Even marine fish live in pH 8.2-8.4 and THAT isn't really super high either so 7.6 is not really a problem. (y)
Thanks so much, I feel so much better! I will be following your suggestions starting tomorrow. I'm so glad to find out about the PH that always worried me. I will keep you posted.
 
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