To better describe my "major water changes"... About 4 weeks ago when I brought the frogs home. I did a quick vacuum, sucking out about 1/4 of the water or less. A week later I did the same thing. 1/4 of the tank. A week later I did what I would call the major water change, when I got the gravel out of there. I would say I replaced about 60% of the water, using stress coat with all the new water. In there somewhere I changed the filter cartridge which is one piece with carbon inside surrounded by floss. My bio wheel spins like crazy after I change that. I thought I had to change it to get the carbon in there. Lesson learned. Certainly cheaper to wash it in old water and put it back in. But it will get gummed up at some point and have to be replaced I assume. I have a pretty good growth of brownish "stringy" algae that hangs out of the tubes where the water comes out of the filter and assume it is all inside there as well. I thought the bio wheel was supposed to be the thing that kept the good bacteria and whatnot...