[bookdragoness]

A little background:

I have a 10 gallon freshwater tank that has been established for about two years now with a plecostomus and three zebra danios (one that seems female, two that seem male). I'd had the same filter on it for most of those, but about a month ago switched to a new filter (AquaClear, I believe).

The female has always done some chasing around of the other two danios, mostly when she was trying to eat the sunken algae pellet.

Today, after performing a water change, I was looking at the fish and saw that the smallest fish seems to be missing a fin and have tearing near his gills: see the fish here

Is this a disease I can treat, or do I need to separate the female bully? What can I do to help this poor guy?

I can take more pictures if it would help. Thanks for your help!

ETA:
If it's useful, here are my pre-water change and post-water change tank readings:
Nitrate: ~80ppm, ~40ppm
Nitrite: 0 ppm (both)
Hardness: very hard (both)
Chlorine: 0ppm (both)
Alkalinity: ~120ppm, 180ppm
pH: ~7.8 (both)

 

[dave in Iliamna]

I am no expert but it looks like an injury to me. I would be conserned about infection. I don't realy know if the other fish could cause this, but I would be tempted to try and seperate him if I could. If he is getting picked on then this might encourage further picking.