I'm hesitant to point this out, as I have no interest in starting another argument about the necessity of bio-media, but because nobody has mentioned it (or I missed it), and cost seems to be a factor in this thread...
Many of us have gone the route of zero bio-media and have perfectly happy plants and fish. While there are a couple valid reasons to use bio-media in a filter, there's plenty of surface area for bacteria to colonize on without adding volcanic rock to the filtration process. Remember, also, that mech-media is *also* bio-media, without exception. Adding bio-media will not increase the amount of beneficial bacteria in the system. It will only provide an area of higher concentration of bb.
Again, there are a couple valid arguments for bio-media, and there are similarly valid arguments to the contrary. Good luck with whatever you decide!