Howdy davi, if you are doing a gravel based flood and drain system you don’t really need a clarifier, but if you are running a raft type system they can be made out of barrels, buckets, bins, trash cans, old fish tanks or whatever you can come up with. My rule of thumb on the size is going as large as you can Aquaponics is addictive and more than likely you will expand. Good luck and please post some photos of what you come up with.