I have a new 55-gallon tank. The chemistry is testing at correct levels (pH, NH3, NH4, KH, GH, NO2, NO3). I have no way to measure dissolved oxygen but I’m sure it’s ok (two under gravel 160 g/hr power heads w/ bubbles, two medium rectangular air stones with deep water tetra tech air pump and a hang on filter unit (non-bio wheel).

I do not believe my tank is creating environmental stress.

About a week ago I purchased (10) Zebra Danios and placed them into the tank. There are no other fish.

After a few days I noticed one Danio with the top-half of it’s tail sheared off. It looked like it was cut with scissors or a razor blade. The injured fish swam differently and tried hiding out by in the rocks.

A few days after the first injury, I noticed another fish with the exact same conditions, the top half of tail missing.

There’s one female Danio that tries to dominate the middle clearing of the tank. She’s about 30% to 40% larger than the rest of the fish. I thought Danios are suppose to be peaceful fish.

Comments?

In my experience, zebra danios can and do chase each other quite a bit but don't do much if any damage. Damage is usually small random rips in any fins. Large rips such as you describe, especially just the top of a tail do not sound like they were caused by the other danios. Since you have no other animals in there, it may be possible that something mechanical in the tank did that to the tail fins. Could they get the fins pulled into or cut off by any of the filter intakes perhaps or an ornament of some kind? Examine your tank with an eye keen for ways a fin could get ripped. Fancy goldfish often get tail rips from being sucked into filter intakes. The same can also happen for small fish like danios, especially if the suction is overly strong or the fish weakened for whatever reason. I hope this gives you some ideas to check into. Then again, the rips could be caused by the danios too and the pattern is just a fluke (pun intended). ;)