If you're talking about a Hang On Back (HOB) Whisper style filter, then what you may be experiencing is your filter getting clogged up. It's normal though, and you should clean it once a week. I do mine with my weekly water change.
Simply remove the filter from the filter tank, unclip it form the spongy pre-filter, and put it in a bucket of aquarium water, and then massage it. (Be careful not to pop open the filter bag and spill your filter media.) If you do it right a bunch of nasty gray stuff will float out of it. This is a week's worth of fish poo and stuff. After you've cleaned it, pop it back in and your water should not over flow anymore... until next week.
In my weekly water change I'll do this, and also wring out the prefilter sponge, rinse out the filter tank, dissemble the intake and clean it with a wire brush, and also flush it out.
I was advised NOT to wring the filter out in tap water, since the chlorine in tap water would kill the bacteria cultures. However, while alot of your bacteria do live in the filter media, most of it is in the gravel, your ornaments, and on the glass. You should replace your activated carbon every month or so anyway, since the micro-spaces in the carbon fills up with the things you're filtering out. That's how it works.