09-03-2010, 08:00 AM
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Technically you don't need any live rock if you are planning to keep a fish only tank. However, as I've come to learn, it will make your life much easier because it offers biological filtration, or as I've read it described, metabolism. Bacteria within the live rock consumes your fish's waste (ammonia) and converts it to nitrite, then nitrate, where it is ultimately converted to nitrogen gas and leaves the tank via the water surface. The alternative is for nitrates to accumulate until you do a water change. You'll still probably want to do water changes even with live rock, but from what I'm hearing you won't have to change as much water as often.
I am in the process of determining whether this is the case myself right now, as I've only recently removed the bio balls from my wet/dry and began adding rock to the tank. I want to add as much rock to the tank as I can while still allowing room for the fish I intend to buy to swim. I believe I've read that it's a good idea to avoid covering rock completely with other rock, though, presumably because the beneficial bacteria is inhibited that way.
I've read the same that Brett has, that you should strive for 1 - 1.5 lbs of live rock per gallon. While it can be costly, if live rock proves to do everything everyone says it does, it will save you a lot of money, time, and aggravation in the long run.