Seagraves,i prefer to add snails/clanup crew in small numbers as algae growth increases.For your glass/diatom algae,you can't go wrong with those Turbo snails.They're better diatom eaters than Astreas and can lift themselves back up when they fall to the sand so your hermit crabs don't kill them.In a 20gal tank though,a couple 5gal waterchanges and the use of activated carbon in the sump/external power filter really helps to lower the dissolved organics in hte tank.However,it also greatly increases the water clarity so it might be a good idea to shade your tank from that MH with eggcrate and a few sheets of window screen.Remove 1 sheet per week and you should be set.