What temperature of the tank?
Increasing temperatures will improve the metabolism and could help the fish pass waste. How often are you doing water changes? How big are all the fish? I don't want to call you a list but I find it hard to believe that you can maintain low nitrates with your tank stocking. Easily overstocked. Even if fish are not full grown.
I suspect a combination of stressful environment, poor water, and possibly internal problem which green terrors tend to have lots of sensitivities.
So many threads of green terror with constipation or bloat or internal parasites ect