Yes I believe most of the reefers here do not have a good imperssion on Seio. The ideal tank size for 620 is 2 1/2 feet and below for best result. It is still possible for 3 three feet tank but will really have get hold on the control of both air flow control and the high / low water flow control. 820 can be used is 3 feet to 5 feet tank. The flow rate for 620 is at 2400lph and 820 is at 3200lph. The electriciy comsumsion is also very low and therefore produce very low heat.Because of the low heat produced, you guys do not have to worry that the pump will like get burned. For marine user, I would suggest that the nozzle being plug out. Only with out the nozzle, will than the water be able to move in circular motion. You do not have to worry that this pump will create sand storm as the current is very gentle. Even the current is gently, it is able to move across the tank size i have mentioned.