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i have an unused tray in my XP3 and i was gonna buy media when i thought i might be able to use this small rocks i have laying around, anyone know if i can do this?

If they're just plain ol' small rocks, they're not going to do much for you, either mechanically or biologically. But they will make your filter heavier
Lava rock works as an OK biomedia, but still isn't great.

If you're looking for some kind of biomedia, check out this site:
http://www.monsterfishkeepers.com/forum ... hp?t=88677

It lists all sorts of different items used for biomedia, and their effectiveness

Well. I was thinking it might act as a prefilter, but it might just obstruct too much water flow.

The only "media" I can think of is the charcoal strips that are usually placed in canister filters. I don't know how you would acquire a safe kind of charcoal, though... Maybe replacement parts at a pet store?