Inherited tank, new fish keep dying (but old residents continue living on)

[Junorhane]

I haven't had a fish tank in decades, but I have recently taken charge of the fish tank at my daughter's school. It's a freshwater tank, 20 gallons, and it has 2 Kuhli Loaches in it that have been in there for years. They hide a lot during the day, so it just looks like an empty tank.

I ran some water tests, and had the local Petco test the tank water, and it seems like everything is within acceptable levels. There is a lot of white mineral buildup on the tank lid and upper edges. I'm not sure what it is, but it's I assume it's coming from the water (which is tap water).

I've been using water conditioner whenever I top off the tank levels.

First I tried buying fancy guppies and putting them in. Over the next few weeks, all five of them died (and we didn't find any bodies).

Next I tried buying a couple mollies and 3 Zebra danios. Over the next few weeks the mollies both died (no bodies), but the 3 danios were all alive for about 5 months until a couple days ago when one of them died (and we found the body).

A few weeks ago bought a couple Neon Tetras and 2 gold platys and all but one of the platys have died (with no bodies).

This is not a new tank. The loaches have been living in it for many years, so I don't want to change too much, but I do want other fish to live in it.

So what is going on? Are the loaches eating the other fish (do they do that)? Is there too much minerals in the water (the white buildup) that is killing the fish? What do I do?

 

[Aiken Drum]

What are your water parameters right now?

The white build up is limescale, the same stuff you get in your kettle if your water is hard. This comes back to what are your water parameters? Some fish prefer soft acidic water, some fish prefer hard water with a higher pH. Unless your water is at one extreme or the other, or you are keeping very sensitive fish it's not likely a water hardness issue.

Kuhli loaches will eat dead fish, so that's where the bodies are going. But they aren't likely to be killing them.

There are many reasons why fish might be dying. There may be an issue with water quality. There may be an issue with how you acclimate new fish to your aquarium. We don't have Petco here in the UK, but from what I hear they have a terrible reputation amongst hobbyists, so you may just be buying unhealthy fish. But let's start with water parameters.

If you don't have a test kit, get one. A liquid test kit like API Freshwater Master Test is a good one to get. In the mean time take another sample to Petco and get them to write the numbers down for you. Dont accept "everything is good" or whatever. You need actual numbers.

 

[Andy Sager]

Check the general hardness ( GH) of the water in the tank because it may be too high for the new fish compared to the water they are coming from.
There is something called " Old Tank Syndrome" where fish that have been in tanks for long periods of time have gradually adapted to "harsh" water conditions while new fish do not because they don't get that long acclimation period. From what you are saying, because the fish do not die instantly when you add them, it's not your acclimation method and it's not the water in your tank being bad. When those things are the problem, the fish usually don't last more than a couple of days. This makes me believe the problem is a high GH compared to the store's water. If this IS the case, you would need to slowly reduce the hardness in the tank water before adding more fish. You can reduce the hardness by using distilled water or RO water in a mix with the tap water to reduce the calcium/ mineral amounts going back into the tank after a water change. Adding real wood decorations can also help reduce hardness over time. ( Another option but not a short term answer.)

A couple of side thoughts: Since not all Chain stores hire the best people, I would suggest have them check your tank water and tap water and ask for the actual numbers. " Fine" and "within reason" are not good answers to figure out problems. Make sure they are using a liquid test vs a test strip as test strips can be less reliable if kept improperly. I would suggest investing in a liquid test kit as well. There are 2 " master" kits on the market, API and Fritz Aquatics master kits that will test pH, Ammonia, Nitrite and Nitrate. You may need to get a separate kit that measures GH & KH. I'm not sure if either company includes them in their master kits.
Regarding the missing fish, yes, Kuhli loaches are scavengers so it IS possible that they ate the dead fish.

Hope this helps.

 

[ghyti]

Sounds like the Nitrates are way too high. Nitrates build up over time and fish in the tank can adapt. But any new fish will not be able to adapt and will slowly die off. Measure the Nitrates, if they are more than about 40 ppm do a water change every day until they get down to about 20 ppm.

 

[Andy Sager]

I thought the same about the nitrates until the test results were said to be "within acceptable levels." so the next step is because of all the mineral deposit at the top of the tank. Those minerals do not evaporate when the water does so it looks like the OP has hard water or possibly VERY hard water ( I had that when I moved to S. Florida. Perfect water for African Cichlids and Saltwater right out of the tap. ) So we really should have all the test result numbers to see what the person at Petco says "are within acceptable levels." and go from there.

 

[Junorhane]

Thanks everyone. I do have a master test kit, and I suspect we also have hard tap water here too (which everyone claims is actually very good water, but I guess maybe not for fish). I also share the opinion that Petco is... not great, but unfortunately I do not have a local fish store.

The numbers according to the test I have are: Ammonia is at 0. Nitrates look to be around 40ppm. Nitrites look like around .40ppm. PH looks like 7.1.

So if the problem is the Nitrates being high, will slowly replacing the water in the tank with tap water lower them? On the other hand, if the problem is the hardness of the tap water, will slowly replacing the water in the tank with RO water solve both issues?

 

[Andy Sager]

Thanks everyone. I do have a master test kit, and I suspect we also have hard tap water here too (which everyone claims is actually very good water, but I guess maybe not for fish). I also share the opinion that Petco is... not great, but unfortunately I do not have a local fish store.

The numbers according to the test I have are: Ammonia is at 0. Nitrates look to be around 40ppm. Nitrites look like around .40ppm. PH looks like 7.1.

So if the problem is the Nitrates being high, will slowly replacing the water in the tank with tap water lower them? On the other hand, if the problem is the hardness of the tap water, will slowly replacing the water in the tank with RO water solve both issues?

The nitrates are not so high to be causing fish deaths but at the upper limits of being healthy long term for the fish so we are back to the hardness being the potential problem. You need to get the hardness ( GH) checked before we start messing with water types. You also should check your tap water's chemistry, including hardness, so we know whether that needs to be altered before using in the aquarium.

 

[Junorhane]

Thanks for all the replies. Very helpful so far. I acquired a GH and KH test. The results are this:
Tank water is 71.6 KH, and the GH test was just... off the charts. The GH in the tank is so high that the test couldn't even measure it.

The tap water, which I've been using to top off the tank, has KH at 89.5 and 143 GH.

I guess what is happening is that the GH is just building up and up in the tank?

 

[ghyti]

The high GH sounds like the problem to me. I have similar water and a similar problem. Fish rarely transition well to my water. Best way to reduce the GH is probably to mix with RO water. The 143 tap water GH is not too bad. Is there anything in the tank that could be increasing the GH, like limestone rocks or a decoration of some kind? I didn't think GH builds up over time like nitrates. A 50% water change with the 143 tap water every day for a while should bring the GH down close to the 143 number.

 

[Aiken Drum]

We should go back to your post #1.

I've been using water conditioner whenever I top off the tank levels.

Are you actually doing water changes (removing water and refilling the aquarium) or just topping up after evaporation?

If you are just topping up, disolved minerals will creep up and up and up. Disolved minerals don't evaporate like water, so when water evaporates and you add new water with its own mineral content your GH increases. If you are topping up significant quantities of water you need to use RO water for this purpose to prevent this increase in GH.

Your tap water GH is about 8 degrees, which is on the border between soft and moderate, but your comment about topping up and GH being off the charts in the aquarium is ringing alarm bells.

Do you have any rockwork that might be contributing to GH like limestone or coral?

 

[Andy Sager]

Thanks for all the replies. Very helpful so far. I acquired a GH and KH test. The results are this:
Tank water is 71.6 KH, and the GH test was just... off the charts. The GH in the tank is so high that the test couldn't even measure it.

The tap water, which I've been using to top off the tank, has KH at 89.5 and 143 GH.

I guess what is happening is that the GH is just building up and up in the tank?

I think we found the culprit.
As Aiken stated, if you have just been topping off the water that evaporated vs actually taking water out of the tank and replacing it, the GH will rise because the minerals do not evaporate. Unless you added some decoration that is calcium based (i.e. Coral or shells or limestone, etc.) , the high GH points to the problem with the new fish. Since those kuhli loaches have been in the water for a long time, they have slowly adjusted to it which means you are going to have to slowly reduce the GH so that they do not go into shock. That means small volume water changes daily or every other day. Since this is a 20 gallon tank, I'd do no more than 1 gallon per day everyday or 2 gallons every other day so that the fish do not experience change too quickly. It's going to take some time this way to get the GH down but it's the safest for the fish. Once the GH gets much closer to your tap water's GH, you can start changing larger volumes of water. Until you get the GH to your tap water's value, I would not add any new fish. Once you are ready to add new fish, be careful not to add fish that require soft or very soft water. I suggest you check with the shop you get your fish from what their GH is so you know how much of a change there is between your water and their water. You'll want to get closer to their water or somewhere in between yours and theirs and acclimate the fish accordingly. You should have better success then.
On a side note, if there is a drastic difference between your tap's GH and the shop's GH, I suggest you look for fish that will do better in your GH so that you don't have to keep chasing numbers.

Hope this helps.

 

[Aiken Drum]

Let's see if I can explain what's going on when you top up following evaporation in the absence of water changes.

Let's say you have a 100 litre aquarium, with tap water that has GH of 100ppm (about 5 degrees of hardness) and you are starting off with that 100ppm GH in your aquarium. Every week 10% of your water evaporates and you replace that water with tap water.

At the end of week 1 your water is down to 90 litres, but has the same amount of disolved minerals, so they are more concentrated. Your GH is now at 111ppm (100 litres ÷ 90litres = 1.11 x 100ppm = 111ppm).

You refill the water with tapwater that is adding more minerals, albeit you are also diluting the minerals in the aquarium a little and your GH in the aquarium drops down a little to 110ppm.

At the end of week 2 your water is down to 90 litres again, but again has the same amount of disolved minerals, so are again more concentrated. Your GH is now at 122 (100 litres ÷ 90 litres = 1.11 x 110ppm = 122ppm).

You refill the water with tapwater that is adding more minerals, albeit you are also diluting the minerals in the aquarium a little and your GH in the aquarium drops down a little to 121ppm.

So in my example, every week topping up the water following evaporation the GH increases by about 10%. If you are only seeing 2% evaporation per week, you would only see the GH rise by 2% a week, but over time those 2%s add up.

Your long established fish have only seen very small increases in GH week to week and have become acclimated to the higher levels and are fine. Your new fish have come straight from water probably closer to levels found in your tap water if you bought them locally, and haven't gone through that months long acclimation process and don't live very long. As Andy says, this is part of a condition called old tank syndrome where people do less water maintenance over time and parameters get gradually worse, fish get gradually accustomed to it, but new fish can't tolerate the poor water quality.

As suggested, unless there is some other factor not mentioned that could be raising the GH (eg calcium based rocks), you need to gradually get the waters GH back closer to your tap water levels. Doing a 1 gallon water change every day for a month should go a long way to achieving this, and then establish a regular water change schedule that will keep water parameters where you want them. Say 30% or 50% every week. A water change involves both the removal and replacement of water. The removal part is important because that's the part that removes the bad stuff, and the replacement then dilutes the bad stuff that's left down a little.

 

[Junorhane]

Thanks everyone. I'm sure you're all breathless for an update, so, here you go: There was a rock in the tank, decorative, I'm not sure what kind it was or where it came from, but I took it out. It's possible it was harmless, but I think it's equally possible some student found it and thought it was pretty and stuck it in the tank years ago and it's been slowly leaching minerals into the water ever since. Anyway, it's not there anymore (and the gravel is store bought, so I'm trusting it), and everything else is plastic.

I've done some water removing and replacing, I think up to about 6 gallons now. I can't get in there every day, unfortunately, but I do try to do something at least once a week. So far the GH still tests too high, but I'm assuming it's slowly going down since the water I'm adding is either RO or spring/distilled water.