This is exactly what I was thinking for you @Tula - a small version of @addy1 's bog would be perfect. The whole pond facing side can just flow back in to the pond. And don't be afraid to go deep to get more filtration - our bog is only 4 feet by 6 feet, but it's 4 feet deep. We have a few Aquablox in the bottom and a layer of softball sized rocks and then it's all gravel. We've had no trouble with clogging or the bog getting smelly (both things we were warned would happen) and it filters the pond great.