just some thoughts...
I've used some sand filters with sand for fish systems. It of course depends on the food given how frequently you need filter maintenence. I have a few fish systems both salt and fresh water running on sand filters with #2 filter sand, the coarse stuff. We got it from Littler River pool and spa in Christiansburg. It works much better than #1 sand in fish systems. Though I agree other media may be more appropriate.
Another though would be to reduce the size of the pump, depending on flow, or add a bypass arround the sand filter, to maintain flow if the filter is clogged. Also be aware that you can end up with supersaturation when running a closed presurized filter system if there is air introduced to the system (leaky pump seal). I can tell you more about that if you're interested.
I have heard, but never tried, that you can use a wood chipper or similar to shread plastic milk bottles for media.
John
