Hi there!
You're right, the painted shells aren't good for them. I've done a lot of rescues the past couple of years, and most of them came in painted shells. The trick to getting them to change quickly is having the right size shell and preferred opening shape. You most likely have a purple pincher, and they prefer shells with round openings. Here is the shell guide to help you out:
http://hermitcrabassociation.com/phpBB/ ... 24&t=92552
Don't go digging for any crabs, but if you can, grab the measurements of the shell opening for those hanging around on top. For example, say your crab has a shell with a 1" opening. I like to get a couple of shells a bit smaller (7/8"), a couple of shells the same size (1"), and a couple of shells a bit bigger (1 1/8, 1 1/4). This way they can size up or down if they want too (or stay the same). As soon as they change shells, I throw away the painted one. A great place to get good hermit shells online is Naples. I've ordered from them a few times and my crabs always like their shells:
http://naplesseashellcompany.com/hermit ... hells.html
AquaSafe will work for your well water. If you ever move and have public water, you would need a dechlorinator that also removes ammonia (like Prime). If you use a 16.9 oz bottle, 2-3 drops should do the trick. Personally, I like to keep a gallon of treated fresh water on hand and a half gallon of salt water. I use those to fill my smaller bottles as needed.
For your tank, you'll want to have at least 6" of sand castle consistency substrate, or sub that is 3x the height of your largest crabby, whichever is deeper. It allows for plenty of molting room. If you have any crabs that aren't on top, you can't dig for them. Instead, you can add 1" of substrate per day, just don't pack it down. Also, don't move any large buried objects. Just place the sub around them. This will ensure any tunnels dug stay intact.
If you can find room, try getting them a silent spinner hamster wheel. My guys love to go for a walk in theirs! You can place it on the sand, or hang it upsidedown with command hooks. Egg crate (or light diffuser), placed on the back and/or sides is great for climbing. You can find that in the lighting section of hardware stores (Lowe's, Home Depot). Usually I place it where I want it to go, and then fill the tank up with sand. The weight of the sand holds the egg crate in place against the walls. If all your crabbies are on top and not buried, you can do this too. I have my hamster wheels attached upsidedown to the crate.
Also be sure to have pools that the crabs can fully submerge in, with easy access in and out (I use craft mesh zip tied on my pools like ramps).
I hope this helps and that I didn't ramble too much lol!
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I'm pretty sure I have more shells buried than there is on top.