It is a tough row to hoe. Overfiltration gives wonderful results but it is more "work" than many ponders are willing to provide. This is where a goldfish-only pond really shines. Koi will keep you (and your filters) busy.
For what it's worth I am not a fan of lava rock. It is heavy and hard to keep clean. In addition it does dissolve (very slowly) and God knows what heavy metals it contains. It's luck of the draw IMHO. Think ceramics.