Agreed on the horseshoe crab. I would also not suggest the cuke
either. The tank still looks somewhat new and cukes as a rule do not do well in younger tanks. Other than water stability, there is rarely enough of a food source to sustain them as yet and would most likely starve.
Here are some good snail choices. Hermits can be beneficial as well but are not "needed" providing the right mix and amount of snails are chosen based on the needs of the tank. If you do opt for hermits, make sure there are plenty of larger empty shells for them to "upgrade" to.
great for detritus and the sand bed.
especially good at film algaes..
cleans both the glass and the sand bed..
cleans up the uneaten food bits as well as detritus.
another excellent algae eater...