ones a 45, ones a 55, and ones a 75, usually about 50% water changed while vacuuming the gravel, i do add chemicals after changing the water before turning the filters back one, but isnt there still a chance of wiping out the bacteria if the chemical didnt work quick enough or well enough, im looking for a cost effective method here. i have yet to fill the 75g, still waiting for the new center brace, but i have been runnning the filters i will be using on that one on the other tanks to try to get them ready for when the 75 is up and running, is there a filter that can hook up to the faucet or something cheap and easy that works