I know I should not really even worry about water changes, still cycling and only on day 4, but I like to be prepared. I have a 30 gallon tank that is cycling fish less. I have read, that once my ammonia & nitrites are 0 and my nitrates go up, to do a water change then add my fish. I have seen some say 20-25% water change. So really how do I decide how much water that is?? I am horrible at math. Do I figure out my tank volume and take 20-25 % of that?? the thought of math kills me. If I think I am doing this right, it would be 6-7.5 gallons to remove?? and of course the bucket I bought does not have measurements on it, it just shows 1-12 inside, but does not say 12 what.