first of, welcome to the best, nicest, and most informative bb on the net! now on to your question. well, it is really up to you. i would reccomend getting a 5 gallon bucket and just filling that up. i have a predator tank and a semi-reef and do a predator water change of 15 gallons(75 gallon tank) every other to 3 weeks, and on my 38 semi-reef i do a 5 gallon change weekly. hth, bi