Depending of the size of the O. scyllarus, 20 gal could be a bit small. It will certainly hold a 3-4 inch animal for a year or so, but I would not recommend it for an animal 5 inches plus.
O.s are sensitive to nitrate. While I can't prove it, I think they may play a role in the fomation of shell disease which is common in O.s.
Skimming helps, but general what is really important, particularly in a small tank, is the removal of uneaten bits of food, molt skins, etc. These will elevate your nitrogen and phosphate levels and given that O.s like to bury such items, in large quantity can be lethal.
Roy