I have a 300 gallon display and a 55 gal sump. The total water volume in the syatem is 280 gal. I add 3 to 4 gal a day. HTH. But what is top off really anyhow? It is only fresh water, it's not like it has the added cost of the salt. My tank has four 10" tubular skylights on it that give it light all day, then in the evening it has three 250W MH pendants that run for three or four hrs every evening.