How do I determine what killed my fish?

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padderson

Aquarium Advice Apprentice
Joined
Jan 30, 2014
Messages
22
I've had a 20g planted and a 10g tank for about 6 months now and both are thriving. My guppies continue to have fry survive and so I fear my tanks will become overstocked. Therefore, I picked up a 20g that someone was giving away for free. I have no knowledge of the tank history, just that they left it on the front porch for me. It had an aquarium background taped on and a single fluorescent lamp fixture, both of which I disposed of...but it seemed to be an "aquarium" setup (versus terrarium or other).



I scrubbed the tank with an aquarium scrubber and warm/hot water from my shower hose (I did not use diluted vinegar or bleach--perhaps I should have). I let it sit in the shower, filled, to leak test it and all was good. I just set it up this past wed with new gravel, filter, and decorations--all pre rinsed. I also moved some plants, decorations, and filter media from my 10g tank. I put in some danios which I already had I the 10g. I'm not trying to start a discussion re fish-in vs fishless cycling...I had these fish already and was planning on the new tank being their long term home. And I do daily water checks and changes when needed.



Within 2 days the danios were looking slow and sick...sometimes gasping at the surface. I did a full water check with the API freshwater kit and everything was fine. I mean, the tank has only been set up for 3 days, so there was no nitrate, nitrite, or ammonia yet. Ph, softness, and alkalinity were standard (I also used the strips to re-check) and all normal for my water and compared to my other tanks. I had used seachem neutralizer and dechlorinator as well as stresscoat and QuickStart when i filled the tank, so chlorine shouldn't be a problem.



Convinced that something was wrong, I pulled the fish and moved them back to the 10g tank where they had started. Within a couple hours they were all dead. The rest of that tank still seems fine. I don't see any other indication of illness/fungus/bacteria so far.



So now I have a set up, empty tank that rapidly killed fish and I have no idea why. I bought ammonia at the store to start a fish less cycle, but even that doesn't explain what killed my fish. I feel there MUST be some contaminant from the previously owned tank. How do i figure it out? Is there some other water test I can do? Can I salvage this set up?



Also, I took a photo of the dead fish. They appear to have some serious discoloration in two spots on their underside. Does this reveal what killed them?ImageUploadedByAquarium Advice1397966912.507314.jpgImageUploadedByAquarium Advice1397966925.146415.jpg
 
Was there any smell to the new tank when you got it eg bleach, etc? How are the plants doing? I'd do a couple of water changes and run carbon to try removing anything just in case.

Next up is they may not have handled the change from one mature tank to a new non-cycled tank for some reason. I guess to double-check but does the ammonia test read 0 for tap, new tank and old tank?

Edit - I can't think of another test. Could be worth checking ph for tap, new tank and old tank as well.
 
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There was no obvious bleach smell, but I am wondering also about what they might have put in the tank.

I do have activated carbon, so I will try that. It's a good idea. Also I'll do plenty of water changes before adding fish again. But I guess there's no way to test for bleach or detergent residue, is there?
 
Not here anyways that I know of unfortunately :( Maybe someone has one. I remember there was one case where water ran over a piece of bike tubing I think it was and into the tank. Whatever was on it, wiped out a lot of the tank so lots of possibilities unfortunately.
 
Tank Cleaning & Fish

Hello pad...

Fish health depends on the water. If the water is pure, the fish and plants will be fine. Tank cleaning should be done with a couple of tablespoons of standard aquarium salt mixed in a gallon of warm tap water and a new, soft sponge. Then, you rinse everything well. It's always wise to set up the clean tank with some floating and let it all run for 10 days or so. You can introduce a few small, hardy and go through the cycling process.

B
 
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