Go with the sand you will regret using an undergravel filter
As for undergravel filters, I've been through this, won't go through it again. Why?
1. Frequent water changes to try to get the ditrus and nitrates out.
2. If you put live rock on it you will build up more dead ditrus spots that will pump out more nitrates in you system. This just makes them simply non reef compatible. They were meant for a crushed coral bottom and are no good with sand.
3. You run the risk of Carbon dioxide poisoning that builds up underneath.
4. Although it can help with the bioload it is only low bio load friendly.
5. With what can seep out from underneath the algea blooms can be spectacular leading to what is called New Tank Syndrome even if you think your tank is seasoned, and if you do not vaccum well you will simply reintroduce the waste back into the water colum. This can reek (he he) havoc on your water chemistry and your PH.
6. Even though you use the best strongest powerheads on it, it will just pull the waste and ditrus down into the substrate traping it till it decays and fouls your water or waits to be removed by you and the vaccum tube. Much better to use sand and good water flow to keep the ditrus in suspension in the water to be removed by the protein skimmer or machanical filtraition.
7. No matter how you slice it the UGF is just a ditrus trap. If anything deserves the name nitrate factory it is the UGF.
Just my OPINION.
Thomas
