Odd Danio Behavior? (Also posted on Freshwater Beginners Forum)

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I just lost one of my male Danios. My tank is in the cycling process (pre-water change, my readings were ammonia--0, nitrite--3.0, nitrate--20 and I've been doing 40-45% water changes lately) and for the past few days he had been acting different. He looked like he was trying to swim downward into the rocks/substrate. I caught him nose-down in the rocks yesterday, and he didn't seem to be moving, so I thought he was gone--but when I moved my finger on the glass, his tail would start moving very rapidly. Today he was furiously "digging" in the pebbles in front of my (fake) rock tunnel, and when I came back to change the tank water he was not there, but there WAS a hole in front one the tunnel leg. I moved the tunnel, and he had burrowed underneath it & died. Now, most of what I read about ammonia & nitrite poisoning mentioned that they would be gasping near the top of the tank, and not burrowing on the bottom.

Any ideas as to what this might have been? Thanks!


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I was just about to post about exactly the same sort of problem - we've been losing danios left, right and centre in our tank to that sort of behaviour. They start going crazy trying to dig into the substrate and darting everywhere, and then die. The only difference with ours is that they die pretty quickly; a few hours, not a few days.

It's heartbreaking because the behaviour seems to be triggered by doing anything with the tank - like putting a net in to get a dead fish out, or a water change. We lost one just this morning getting one that had died the night before out. I'm honestly at my wits' end, because I'm almost too scared to get a dead one out in fear it'll kill another. I'd really love to know what could be causing this.
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