I have 2 350's in my 55 gallon and I wash all 4 filter pads at once. never had a problem yet. Newman is right though, don't scrub them to hard or u will rip a hole in the floss. I use well water and I do mine in the shower with the shower head, lightly scrub them and tap them out. once brown goop stops coming out of it and it feels soft again I know it's clean. I should clean them more often though since I wait till the water starts flowing out the overflow.