the rate of flow from your return pump depends on several factors, 1 what height are you pumping water to, 2 how many 45 degree elbows are there in the return line and 3. what kind of pump is it. you can usually find this on the spec sheet or online about the rate of return . the 1300 gph i`m guessing is from powerheads?. dividing that by 72( the tank size) gives you a 18x turnover rate, that maybe enought for what you currently have in your tank.