you can have it as high or as low as you want. If the baffles are set too high you will get an over flow. Thats why in my drawing my baffles dont go to the top. You need to figure out how much water will drain out of your tank when the return pump is off that will tell you how high the baffles can be. That size tank sounds like a 40 breeder that would be fine a 50b would be better 36x18x18.