FYI, sand sifting stars and fish actually eat the benifical critters that actually clean the sand.
The best critter for nano tanks are nassarius (vibex are smaller or tonga for bigger) snails, cerith snails, bristle and spaghetti worms. Cucumbers are good for bigger tank, not a 20. Best to keep your sand one inch or under, and carefully vacume it to maintain. dsb (4"+ of reef agaronite, 2.5"+ of sugar fine)don't work for smaller tanks as its not enough volume to sustain enough of the microfauna to maintain itself.
Also, if you get hermits at all, I only trust scarlet reef crabs. Others much more likely to eat snails and cause trouble. Plus mine roam my sand bed all night while they graze on my rocks during the day