<< that is what my mom did. get one of those bacteria eating fish too...sucker or whatever. they help keep the tank clean. >>
A Plecustomus (yeah, I can't spell it) Does not eat bacteria. It does not eat leftover food, and it does not eat other fish's crap. ALL it eats is algae. Algae is NOT poinsonous to your fish. In fact, lots of fish like it. The ONLY bad thing about most green algaes, is that they are ugly, as they grow on the sides of your tank. If you scrape the algae off the sides of your tank, and have pleco's, they will probably starve if you don't feed them something else (they make little compressed algae disk things for them)
Lots of people think that these fish eat anything that other fish don't eat, and clean up the tank. They don't. If you want a fish that will eat leftover food that sank to the bottom of the tank, get a catfish. If you want to get rid of fish sh!t, get a good biological filter, cause fish won't eat it.
Anyway - you should never completely change the water in the tank. Fish get used to the water how it is, they're very senstive to water quality. I used to change 25% of the water in my tank every two weeks. (I don't have fish any more). Also - you don't want the water to be room temperature, you want it to be the temperature of the water in the tank (in tropical tanks, which is basically anything but goldfish - this is usually between 75 and 80 degrees)