Couple questions. How long have you had both jacks and what size were they both when you acquired them? Also, what other fish do you have in the tank with them, if any?
Keep it up with the water changes, that always helps keep any fish healthy! I find that a greatly varied diet helps also. I feed mine bloodworms, brine shrimp, good pellets, flakes, occasional crickets and earthworms, shrimp pellets, occasional beefheart, and probably other things i cant remember at the moment. Of course all in appropriate amounts, usually twice daily.
Jacks tend to be a pretty aggressive fish amongst similar sized cichlids, and can be quite temperamental also. If they feel they are being "bullied" sometimes that can cause them not to eat. And sometimes it doesnt seem like they are growing but they may just be growing at a slower rate than you expect them to. I got a new jack from a local breeder one time who was a three inch male, and the guy claimed he was a grown adult. (Over a year old). He was beautiful, so i brought him home and he didnt seem to be growing at first either. But after a few months i started noticing a difference, and before long he was up to 6-7 inches and as healthy and beautiful as can be. Have a little patience! Hope this helps
