Ditto @aussieJJDude's comments! Thank you for researching these little guys before taking the plunge -- enough "weird stuff" will happen after you start crabbing so it's best to have a good idea of what to expect from the outset.
If you will be getting the crabs from a standard pet shop (i.e., Petco, PetSmart), you will most likely find Purple Pinchers (PPs) as they are the species harvested closest to the US in the Caribbean. Occasionally, a few Ecuadorians (Es) find their way to a Petco or PetSmart but PPs are the norm. Just be aware that Es are more active than PPs (think Daffy Duck vs Donald Duck), require higher temperatures, deeper substrate and, on the whole, larger tanks than PPs.
PPs prefer shells with an "O" opening, such as turbos, while Es prefer shells with "D" openings, such as babylonias or whale eyes. The easiest way to get an immediate assortment of both O and D openings is to pick up a bag of mixed shells in a craft shop such as Jo-Ann, Michaels or Hobby Lobby. A bag will be under $10 and you should be able to get 6-12 usable turbos (green shells) and about the same number of D-opening whale eye, babylonia and murex shells. The bag will also contain quite a few flat scallop-like shells which are great as food dishes for small amounts of fresh foods. Once you have had the crabs for a while and they've undergone a molt, you can then shop online for the sizes they'll need. Naples and Richard's SeaShells, listed on the Shopping Resources page, are excellent and MyGann on ebay is widely used by many HCA members.
That said, the way to optimize the space in the tank depends on which 20g you have. There are two 20g lengths: the standard 20g is 20" long while the 20L is 30" long. Both are fine as a crabitat but the 20L provides more area for the crabs to tunnel under as well as more space for pools and decor. Second levels are loved by all crabbers and crabs: we love having the extra space for fun features such as moss pits and suspended shell shops while the crabs love having more places to climb to
. The easiest ways to create access to upper levels are to use plants/vines, fish net, small branches/cholla or the old reliable, plastic craft mesh. Check out your local dollar store and thrift stores for plastic or cloth vines and plants for the crabs to climb, sleep on and hide under. A brightly colored cloth flower will also look great in the tank and, with its long plastic stem, it can be poked right into the substrate! Fish net and plastic mesh can be found at Jo-Ann, Michaels and Hobby Lobby: the netting is bagged and runs about $6; the craft mesh comes in a variety of colors runs under $1 for a 10"x13" sheet and is also terrific to use as pool ramps.
Hope this helps!
) In there, you will find appropriate shells that hermit crabs will love, crabby care and ideas in creating the perfect tank!