Question??? I have (17) 2 lb. Tilapia in a 300 gallon tank. Last night I went to feed them and one was on it's side with a blood red discoloration on both sides like wounds close to the surface. I straightened him up and he started swimming around again. A couple others had the same battle marks. Some had what appeared to be small amounts of skin hanging on the sides like abrasions or scrapes. One even had a small chunk of skin hanging on his head right above the eye like it had run into one of the cinder blocks. I fully expected one to not make it thru the night, but all is well this morning. The only thing that has happened in the last two days, is I added 20 gallons of water to the tank and added a new heater 6 days ago. Nothing out of the ordinary. Has anyone seen these type marks before? Looks like maybe they are doing battle, but I haven't seen it. Their behavior seems normal otherwise...
I would add a bit of salt to help them heal their wounds
Do you know how warm the water is? I remember when I tried to breed some tilapia that the males can sometimes get overly aggressive with the females when the males are in the mood but it isn't really warm enough yet for the females to be really ready to breed. If the males are really chasing the females around it is quite possible that some fish are banging into the rough edges of the blocks and the males will sometimes also bite at the sides of the females .
Now I never had the tilapia domestic violence in the larger tank that I know of, I also didn't have any blocks or hides in the tank for them to bang into.
Here is some info which may or may not help.