What wiped out my fish in the span of 2.5 days?!

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Kaynaydian

Aquarium Advice Newbie
Joined
Jul 22, 2021
Messages
9
Location
Ontario, Canada
I have a 10g tank that has been established for several years. It has an Aqueon QuietFlow 10 filter. Tank temp is 72 degrees. Consistent GH 180ppm (we live in an area with particularly hard water), KH 120 ppm, PH 6.5-7 as tested with API 5-in-1 test strips.

For most of the pandemic, the tank contained one clown pleco and a lonely neon tetra (the others died gradually over the course of several months) since the local fish stores were closed to in-person (aka live fish) shopping for most of the year.

Fast forward to two weeks ago - water parameters were stable, but I did replace the filter cartridge and do a pretty thorough cleaning (vacuumed the gravel, cleaned the plants and rocks) and did a ~50% water change in anticipation of adding new fish to the tank.

Bought 2 guppies and 3 neon tetras to add to the tank. All was well (despite one guppy not doing so well.) Nitrites and Nitrates did spike somewhat over the next couple days which I expected, given the addition of the new fish and I did daily water changes of ~25-30% to keep the nitrites and nitrates down enough so the bio cycle could get caught up. Lost the one guppy (which wasn't surprising because he was pretty small and seemed really stressed from the get go.) All other tank inhabitants were doing great! My original neon loved having new friends, the other guppy was lively. Pleco was even more active.

A few days later, I was getting readings of zero nitrites and nitrates. I should have expected something was amiss at this point, but I didn't. I continued to do daily/every other day water changes though.

I also added a new waterproof/underwater LED light to the tank around this time.

Suddenly, on Wednesday, I lost all 4 neons in the span of several hours. They went from being fine to dead. Some erratic swimming upright, then swimming on their sides before ultimately dying.

I checked water parameters again. Hardness and PH was unchanged, zero nitrites and nitrates. At this point I'm starting to get suspicious that the tank has NO nitrites or nitrates. I check the Ammonia levels. Ammonia tests at .25ppm. Nothing outrageous, but not great. I continue with 30% water changes and added a low dose of aquarium salt - 2 tsp in 10g.

The following day the pleco was dead. Still no sign of nitrites or nitrates and ammonia is ~.5ppm. I'm starting to suspect the ammonia was the silent killer in the tank, though it was still testing fairly low. The lack of nitrites or nitrates is a concern. Again, a water change and I added Aqueon Pure in the attempt to boost the bio bacteria.

This morning, the guppy was fine though lethargic. A few hours later, he too was dead.

Water tests again showed the same hardness and PH, zero nitrites and nitrates, and .25ppm of ammonia (which should have been neutralized though obviously still present by the Aqueon Pure by this point.)

What killed my fish and the beneficial bacteria in my tank? Was it the ammonia? I should be seeing nitrites and nitrates right? I rinsed the LED light before putting it in the tank. Is there any way that contributed to the fish loss?

The fish themselves had no outward signs of illness and went from being relatively fine, to slight lethargy to dead very quickly. I'm at a loss.

Any insight is greatly appreciated.
 
I am still a newbie here but why did u have to change the filter and water and cleanout everything? from what i read, u shud keep the old filter with new filter and run it for a few weeks before letting go of old filter is the way to go so that new filter gets established. Dont u think you lost the beneficial bacteria and may have to cycle the tank again?
 
I agree with the above comment about filter cartridges. Its possible you crashed your cycle by doing this. You should only change them if its absolutely necessary. Just periodic rinsing.

Does your filter hold 2 cartridges like my small tank filter does? If so never change them both at once.

2 weeks is a typical timeframe for ammonia poisoning to result in deaths. A sudden drop in beneficial bacteria and a sudden increase in bioload resulting in an ammonia and / or nitrite spike.

Is it possible you forgot water conditioner with your water change?

Might your water proof light not be so waterproof?

Any possibility of soap or detergent getting in the water? Hand sanitiser must be a much more common household object than it was previously.
 
It really sounds like you crashed your cycle.. you should never clean plants, substrate etc... vacuuming gravel and water changes should really be the only hard-core cleaning you do inside the tank after its been cycled. Water changes don't ultimately effect your cycle because the water column doesn't matter much unless it's to remove nitrates. Nitrates being at 0 is a problem. A cycled tank will have nitrates, and no Nitrites. The ammonia spike is normal when your tank is starting a new cycle, it will go down and nitrites will spike then drop and then your nitrates will build back up. When they do, and your ammonia is at 0 with nitrites, your tank is cycled again.

I'm also new to the hobby. But I know enough to know you've crashed your cycle... Filter media should be left alone unless it's over loaded, but even then you'll just rinse it in your old tank water when you do your regular water changes and put it back.

You might wanna add some bottled bacteria with prime to get your cycle jumping again.
 
Thanks for the insight everyone.

I definitely didn't forget to add water conditioner when doing the water changes. Though I did run out of Prime and switched to a Fluval water conditioner that doesn't neutralize ammonia like the Prime does.

To my knowledge no soap or chemicals ended up in the tank. I have a designated bucket I use just for aquarium stuff. The rocks and plants I just moved so I could vacuum the gravel, then put them back in the tank.

The tank itself was pretty clean but I had a lot of white hard water residue around the top, filter and lid from condensation. And the filter was pretty nasty with a lot of brown sludge (which I understand now is actually a good thing?) Ugh.

Apparently my more negligent aquarium keeping was actually better for the tank than overly fussing with it.

My question now though, shouldn't I be seeing some nitrites by now since even if the cycle crashed it should be starting to re-establish. It's been over a week since I first had zero nitrites/nitrates. And especially since I added the biological boost via the Aqueon Pure?
 
My next question...

Since I clearly don't have a bio cycle or fish right now, do I continue to try cycling this tank that has ammonia and the Aqueon Pure in it and see if it cycles? Or do I start from scratch with all new water, clean filter, etc in the event that this was all caused by some mystery toxin or ailment?
 
Thanks for the insight everyone.

I definitely didn't forget to add water conditioner when doing the water changes. Though I did run out of Prime and switched to a Fluval water conditioner that doesn't neutralize ammonia like the Prime does.

To my knowledge no soap or chemicals ended up in the tank. I have a designated bucket I use just for aquarium stuff. The rocks and plants I just moved so I could vacuum the gravel, then put them back in the tank.

The tank itself was pretty clean but I had a lot of white hard water residue around the top, filter and lid from condensation. And the filter was pretty nasty with a lot of brown sludge (which I understand now is actually a good thing?) Ugh.

Apparently my more negligent aquarium keeping was actually better for the tank than overly fussing with it.

My question now though, shouldn't I be seeing some nitrites by now since even if the cycle crashed it should be starting to re-establish. It's been over a week since I first had zero nitrites/nitrates. And especially since I added the biological boost via the Aqueon Pure?
If you have completely lost your cycle, i would expect a couple of weeks before you start to see nitrite and maybe another couple of weeks from there seeing nitrate.

As for the aqueon pure, im aware of the product but have no experience of it, and never seen a review of it either positive or negative. If its like all the other products that make the same claims, then i wouldn't bank on it have benefits. If it really worked, would it be more commonly used?

I would possibly get a second opinion on your water parameters. Local fish stores usually do water tests and some do this for no charge. Or try a different test kit.
 
My next question...

Since I clearly don't have a bio cycle or fish right now, do I continue to try cycling this tank that has ammonia and the Aqueon Pure in it and see if it cycles? Or do I start from scratch with all new water, clean filter, etc in the event that this was all caused by some mystery toxin or ailment?
If you want to do re-cycle the tank you need an ammonia source. Either dosing ammonia and do a fishless cycle or adding some fish and doing a fish in cycle.

Im not sure what to advise as to starting over. Probably best to change all the water and clean the filter (rinsing rather than super cleaning). Are we are in the realms of a bleach solution disinfection to make sure anything is killed off? We arent really sure what killed everything, just best theories. Maybe someone else will chip in.
 
Thanks for the replies Aiken.

I haven't done anything with the tank yet. I'm going to take a water sample to my local fish store and see what they suggest. Whether I end up with a couple guppies or something to try to cycle this tank as is, or I head home, scrub out the whole thing and start fresh.

I am a bit concerned because I have been doing water changes regularly which should have prevented really toxic ammonia build up, I'd think.

My other guess is that LED light. That's the only thing different in the tank, and the fish all seemed to die when it was on. But it doesn't seem to be affecting the water temp or anything so I'm really at a loss here.
 
I agree that water changes should have kept ammonia and nitrite at relatively safe levels. Was you testing in the period immediately after you introduced your fish, a couple of weeks before they started dying? You mention a nitrite spike, what did it spike at?

The symptoms you mention dont really match common ammonia or nitrite poisoning symptoms, but nitrite poisoning causes organ failure, so i guess that could result in a wide range of symptoms. Erratic swimming can be a symptom of ammonia poisoning. Did you notice any gasping at the surface or discolouration especially around the gills?

A thought that comes to mind. Are you sure the underwater light isnt a UV light? I now remember someone on here mistakenly using a UV steriliser light intended for a sump in their tank and killing all their fish in a similar timescale you mention.
 
If this light is a UV light and not LED, I swear to god.... (It looks like an LED and it states it's for aquarium use, but I bought it off Amazon so who the heck knows.)

I don't recall specifically what the nitrite/nitrate levels were, but definitely pink on the test strip. Which prompted immediate water changes twice a day to get those down. But to go from a spike to then nothing... it just seems odd. A UV light on for 8 hrs a day would certainly do it! And would explain the crash of the nitrogen cycle.

And no, the fish weren't gasping for air at the top or anything. Just some erratic swimming at least for the neons and lethargy in the pleco. The guppy was pretty active from the get go but he'd also sleep/sit on the bottom of the tank (which scared me at first but upon research isn't abnormal for them and since he did that consistently since we got him, it didn't seem illness or toxicity based.)
 
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Your light being a UV is unlikely. Im just throwing it in the mix as a possibility as i remember it happening to someone.

The sudden drop of nitrite and nitrate is certainly odd. I wonder if there is an issue with the test kit. You use strips?. Would getting a liquid test kit be out of the question? API freshwater master test kit covers what you need, is easy to use, more reliable than strips, and as you get 100s of test from it more cost effective long run than strips also. Getting your fish store to do a test is a good idea, but make sure they give you the numbers rather than saying they are "good".
 
Yes, I use the 5-in-1 strips, though I have the liquid ammonia test. It's definitely several years old though. I bought a new package of the strips (thinking maybe the others were old, etc) and they show the same readings.

I'll definitely be asking questions at the LFS (and the one that specializes in fish, not the chain store) so I'm sure they'll give me exact numbers.

Thanks again for your insight, Aiken. It's appreciated. I've had fish off and on for decades but I'm more of a dabbler/hobby fish keeper than an expert.
 
Had water tested at the local fish store. Water parameters were perfect. 20ppm of nitrates, 0 nitrites and 0 ammonia, pH 7.0. I didn't test the water myself before going to the fish store, but testing just now and sure enough, 20ppm nitrates, 0 nitrites. So I guess the Aqueon Pure started working? Though it managed to convert from ammonia to nitrites to nitrates pretty dang quick. *shrug*

The guy at the fish store was stumped too. The only theory he had is that the neons succumbed to Neon Tetra Disease, which could have stressed the pleco (who was also dealing with more light than he was used to) so he died, and the guppy then died because all his tank mates were suddenly gone.

Bought two new guppies to see how they do and make sure the tank is fully cycled before adding anything else. If these guys die, then I'm back to the drawing board and will clear out the whole tank and clean everything with salt water and start fresh.
 
But you havent been putting ammonia into the system so there has been nothing for the aqueon pure to work on to get those nitrates.
 
I hadn't added any ammonia, no. But there was still some of the neutralized ammonia in the tank. Now that ammonia has been converted to nitrates since it's at zero and the nitrates are up.

The tank was only without fish for a day. Now, the two new guppies seem to be doing well so far! *fingers crossed*
 
I am quire sure you killed whatever biological filtration you may have had by replacing your filter cartridge. Given your aquarium had just 3 fish for so long meant the bacteria colony was minuscule since it will grow or shrink depending on the number of fish and tank size.

Mt advice is to recycle your aquarium and start again by only adding 2 fish max per week after cycle is completed. When you ant to do housecleaning it is best to keep your filter cartridge and simply rinse it clean in a bucket of water you have siphoned from your tank. Keep in mind your filter is the main source of your biological filter because all the water in your tank will eventually pass thru it and provide the largest source of food to grow the colony.
 
I think so too, Talon.

But my cycle seems to be getting established again and I'm getting nitrates with zero nitrites and ammonia. The two guppies I added to the tank are doing well.

I'll give it another week or so and add some more fish then - likely 3 corydoras.
 
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