A co-worker gave me her daughters 30 gallon tank yesterday. The kid is just too busy with high school and sports teams. It had three remaining survivors in it. A zebra danio, a scissor tail and one beautiful big old peppered cory. They are all 5 + years old.

The danio has a red inflamed protruding gill. The water quality was bad. Mom, my friend had a fluval 2, and a top fin 40 in the tank. The fluval got clogged and was not working (is now) and the HOB top fin 40 was going, but the only media it had left was one small sponge.

There was decaying dead fish.

My question is, what is this, is it contageous, etc...