Found a fish with an abdominal swelling.

The volume of the aquarium is 300 liters (length 2 meters, height 35, width 50)
Launched 3 months ago
13 adult fish live in.
There is no light
Temperature 29-30 degrees
Two Eheim Professionel 3 2080, three Tunze pump flow pumps at 6500 l / h
No soil
The ...

Thank you for your great advice!

Hello everyone!
Using the aquarium Söchting Oxydator W for over a month. I am pleased with the result: no noise and tubes and oxygen level is 6 mg/L.
The water in the aquarium is about 150-200 liters. L046 live there and I read that they prefer more than than 8 mg/L
Now I use a 6% hydrogen peroxide ...