Hi, I currently have one 3" Jack Dempsey, one 4" blood parrot, and one 3" green terror in a 4 ft. 55 gallon tank. I'm not looking to upgrade this tank anytime soon due to room constraints, so I was wondering if these fish will be OK in here long term...I have a Penguin Biowheel 350 (350 GPH) and due weekly 25% water changes. I was also looking to add possibly a few smaller dither fish to "fill out" the look of the tank and reduce any aggression, if it arises. So far all of the fish are getting along fine. Is this tank overstocked? Also, any recommendations for smaller dither fish?