My dentist has a large hex tank maybe 90-120 gallon. Looking closly at it, I saw no filter other than a couple of bubble stones hanging. I asumed it's an undergravel filter but non of the lift tubes have power heads or air stones. The gravel is was mostly bits n peices of shells about 3-4 inches high. The Tank houses a picasso trigger, an angel fish, and several other neat saltwater fish.. Can someone tell me what might be going on and how this is possible. I did notice one of the fish (angel) i think had scabs on it's face and the others looked ok. One of the air stones was like a big circle pad and the other just a small stone, both just hanging there? Any Ideas?