Sounds like bloat. Try treating them with Jungle Parasite Clear, asap. Do a 50% water change first.
On the main tank: do a 50% water change with a good quality declorinator and siphon the gravel and then since bloat is contagious you really should treat that tank, too. Even if none of the fish are showing symptoms.
Ongoing aggression is stressful for your fish and may be partly to blame for allowing your fish to get bloat in the first place. Also 40ppm of Nitrate is high. Yes, nitrate isn't as harmful as ammonia and nitrite but continual high levels will weaken your fish and leave them susceptible to disease. Try doing more frequent partial water changes. (30%/weekly) Make sure you're using a good quality declorinator, one that removes ammonia and detoxifies nitrite. Prime and Amquel Plus will do it but there are a few others.
Robin
On the main tank: do a 50% water change with a good quality declorinator and siphon the gravel and then since bloat is contagious you really should treat that tank, too. Even if none of the fish are showing symptoms.
Ongoing aggression is stressful for your fish and may be partly to blame for allowing your fish to get bloat in the first place. Also 40ppm of Nitrate is high. Yes, nitrate isn't as harmful as ammonia and nitrite but continual high levels will weaken your fish and leave them susceptible to disease. Try doing more frequent partial water changes. (30%/weekly) Make sure you're using a good quality declorinator, one that removes ammonia and detoxifies nitrite. Prime and Amquel Plus will do it but there are a few others.
Robin