Help I can't figure out what is killing my fish! Please Help!


New Member
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5 to 10 years
Please help I don’t want anymore of my fish to die.

To begin I have a 56 gallon tall planted tank that had 6 rummy nose tetras and 7 black neon tetras. I also have a 75 gallon aqueon quiet flow filter on the tank. I do monthly 20% water changes.

Two weeks ago I did a water change. I also wanted to slightly change the look of my tank so I added some large black pebbles and small white rocks that I had left over from gardening.

4 days ago I received 8 new neon tetras from Live Aquaria in the mail I acclimated by floating the bag and slowly adding water. After the first night 3 were dead I scooped them out of the tank in the morning. Then 3 more died the next night. Then 2 more died. I always scooped them out in what I assume was at most 5hrs since they died. I was heart broken but assumed I might have gotten a bad batch.

Then this morning I woke up to a dead black neon that I had had for more than 2 years. Now i haven’t had a fish die for more than 2 years so I slowly started using my water test kit less and less until i forgot about it. Today I used it and the colors were no where near the guide so I assume they must have expired. I panicked and did a 35% water change.

Tonight another black neon died.

I am clueless as to what is happening. My only guess is that maybe the black rocks I added could have affected the water quality. The white rocks say aquarium safe on their bag.

Here is a pic of the black rock

Should I put any medication in the water in case they have a disease? I can’t see any symptoms though.

All help is greatly appreciated

TLDR: I added new rocks and new fish to aquarium. All new fish died. Now old fish are dying. Are new rocks toxic? I don’t see any disease symptoms should I medicate?


Well Known
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San Diego California
2 years
You have 2 issues maybe 3
1,doing monthly changes @20% is not sufficient enough to sustain a healthy pristine environment for your fish, weekly 25%~50% is a common recommendation on this forum.
2,rocks in the garden will hold on to ferts and soils not meant for fish and will leech into the water, rocks should be boiled and or scrubbed with hot water and placed into a 5 gallon bucket with source water and prime added. Leave them in the bucket 24 to 48 hours amd test the water to see what has changed or any high levels of anything.
3, possibly your fish you just bought were not compatible in the system ie. Ph level,temp?, water hardness things like that.

I would stress the water exchanges strongly and leave the filter media alone so beneficial bacteria can thrive.

Get a test kit and check the water levels.
Take the rocks out....all of them that you added fish safe or not.
Do a big water change right now and see what happens over night.
Sorry this is happening to your fish.
I hope you do not lose anymore.


Well Known
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More than 10 years
It’s hard to tell if the black stones are coated or natural. In any case it would be best to leave them out. However they were in the tank for 2 weeks before you had any deaths which only started with the addition of new fish. My guess is that it’s related to either disease or poor water quality. I would change more water, at least 50%. 20% monthly is not enough, you should be doing at least 30% weekly. It’s possible that you have high nitrates and the new fish pushed it even higher which caused the deaths in both the new and old fish.

For now I would get a new test kit and post the results and observe the remaining fish closely for any odd behaviour or signs of disease. There’s no point medicating if you don’t know what you’re medicating for.


Active Member
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4 years
I am sorry about your fish dying. I agree with Wrench on every point. You added new fish, and the rocks without quarantining. You do need to up your water changes because in my opinion your test kit was sending up a huge red flag. Did you test the kit on your tap water? That would tell if it is working.

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