Aside from gas, it might be dietary or how much that poly likes to eat. Bring this up because I get my 17"+ cb endli that will float up slightly on the back end 1-2" from time to time. It's from overeating of cut tilapia. It only happens on occasion.
Moke its usually last one to show up for feeding so i have to make sure there is enough for her to eat. On top of that my chocolate likes to stove herself with food. I dont realy measure how much i feed. They get carnivore and cichlid gold on alternate days plus tilapia every 3 days. Fasting on fridays