Matt talked about using a polyfilter at the drain. Please explain. My system is set up with a 2" bulk-head with 2 90 degree arms, one that turns up towards the water's surface inside the tank and one that turns down and connects to my tubing that drain into my refugium below the main tank. It drains first into a small overflow box. So where do I put the polyfilter (what is a polyfilter?). I also don't have a canister filter so where is the next best place to run carbon? Can I put it in a media bag then put it directly into the overflow box?