You are going to need a solids filter (pre-filter) otherwise your biofilter won't be worth anything. Some of the pond filter systems out there are not very well suited for RAS because the fish and waste are not concentrated and the pond's biological process takes care of it's share of the process.
A bag filter would be a good choice for something simple and cheap. Plus you can clean them easily for reuse. They will eventually become particulate laden, but they are cheap.
What fish are you planning on?
