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?