If i create a 10" deep, 12" pool at the top of my water fall and then have the hose that pumps water into the waterfall submerged in that said pool - would the higher water speed that comes directly out of the hose at say 500 gph be too strong for any effective biofiltration? Would my bog idea of water flowing 1" above a 4" substrate of lava rock bog which is 4' by 4' (flow coming from waterfall and the bog empties into the pond) - would that be very effective. Im gonna keep a lot of tadpoles and frogs in the small 600 gph pond so i want to design an effective, natural biofilter using the right combination of circumstances dealing with the waterfall as oxygen source and gravel and rocks as the medium for colonization. Could you give me some suggestions? I was also thinking of having a culumn of gravel behind the rocks in the water fall so that water will be spread out and flow over them, but again, as for my initial question: im afraid that the water flow will be too direct. Also, as for my small pool idea at the top of the water fall: the water will be comming from the pond and will not have created oxygen so i dont know if thats the place to expect a lot of biofiltration.

Right where the flow comes out, it will be fast but will soon dissipate. Not much will stay put at the immediate exit location but neaby, it should be fine. If you can get the water inlet to come in under the lava rock substrate, that would create an upflow biofilter if you want to try that. The waterfall should aerate the pond water so the exit water should have a good amount of oxygen still there.

Anyone else have ideas?

i think this response (that i previously missed) answers the question that i just asked in a different thread. thanks.