I built one like that about fifteen years ago with a shower head and a five gallon plastic paint pail for my discuss tank. It was one of the best filters I ever made. I used lava rock mulch for the media. After about two months I decided to do a water test prior to a water change to see how my nitrates were. They were impossibly low. Barely detectable as I recall.
I figured that there must have been anaerobes living at the center of the lava pieces eating the nitrates! I tore the tank down a couple of months later because I got transfered through work and never did figure it out completely. How do you deal with nitrates on this tank. I don't imagine they're a problem in your green house...