Your phosphate, nitrate and calcium scare me. At those levels of NO3 and PO4 I'm not at all surprised that you have algae and slime problems. I am surprised that you dont have calcium precipitation.
Marine Invertebrates: 500 Essential.... By R. Shimek provides some excellent insight on keeping snails and The Reef Aquarium Vol 3 by Delbeek and Sprung is the best book I've read on aquarium chemistry and setup. Or spend some time reading through
www.reefkeeping.com and
www.advancedaquarist.com (two online magazines) for some detailed direction on how to get you system healthier.
+1 for larger weekly water changes.