i run hot water into a bucket from the main bath tap and top up with cold to get the right temp, add de-clorinator, have a big stir, and pour slowly into the tank. the guppys and cardinals think it's play time when i do this each week! instead of getting scared and hiding, they relish the new water and RIDE THE WAVES MAN!! not only have i cleaned crud from the tank, but i have given my fish a bit of fun once a week, after all, they give me entertainment every day. today tops it all..... i watched one of my guppies give birth....! wow!! anyway, the point is, syphon the crud/crap out and give them fresh water, they will love you for it!