I clean my filter every 4 weeks, and all I do is rinse the media in old tank water until it's clean. Don't replace it unless it's teraiong apart, and if I do it's max 50% of the media.
I had that same problem when I was keeping tow goldfish in a 20 gl tank, with a Whisper 20 Power filter. I had to clean the floss every two days bacuase the water would ovwerflow. Sometimes, it indicates your filter is not up or too small to handle the bioload.
Why don't you try prefiltering the overflow? don't know your exact setup, but this could prevent a lot of poop going down the stream.
My 180g is an AG, with built in corner over flows. I think the pre-filter sponges avalible would be hard to clean. Just as easy to pull out he sump tray and slap in new padding.