The trick is to find stuff that has been dead for a while. If it looks like it's been dead for a while and is still dense and hard, it should be fine.
Are you looking at live stuff? Is it really branchy and angular like Manzanita? If it is then you are likely looking at Strawberry Guava. Manoa Cliff trail has a lot of it. I've used it and it is fine in the aquarium. Just make sure you dry it out well. The other tree that has good structure up there is Ohi`a. The bark is really rough, usually covered with lichens and mosses, the leaves are smallish and round, and it has a nice red flower. It is a really hard wood and although I have never used it personally, it should do fine in the aquarium. I stay away from the stuff because it is becoming somewhat rare and it has cultural significance to native Hawaiians.
If you do plan on collecting wood up in the mountains, be discrete about it as I don't think it's legal. Could be wrong though.