What we've found with our pond is that the mechanical filter traps fine gunk at the surfaces immediately facing the incoming water (dead algea mostly, the pump intake prevents anything over 1/8" from getting in. Leaves just go straight to the bottom.) Given what you did with the waterfall, you could at some point substitute one of the filter levels with chuncks of lava rock. That'll give ammonia eating bacteria a place to grow.
Our pump is a bit undersized (3500 gal pond/ 2500 gal/hr pump) but even with that, everybody is happy (fish, frogs, tadpoles, salamanders, ...) without needing any water treatments. OTOH, we do only have four fish (smaller koi) at the moment, not 10 good sized ones.
Just curious, what kind of fishees did you end up with?