Land hermit crabs require access to both fresh water and ocean water at all times. This need is met by providing two water dishes or pools that are deep enough for your largest crab to submerge themselves.
Why do some sources say you should bathe your hermit crabs once a week?
This is one of those old practices that will not die. The reason is a bit convoluted though. Retailers who care only about pushing products and promoting hermit crabs as ‘cheap and easy’ pets refuse to acknowledge that hermit crabs NEED humidity to breathe and therefore survive beyond a month. 90% of the hermit crab products sold do not meet the needs of the hermit crab. Is it any wonder they die after a month when they are denied basic necessities? Retailers only care that you bought the garbage products to begin with. Anyway, instead of demonstrating how to set up a proper habitat with correct humidity and water pools they tell you to mist your crabs twice a day and then once a week put them upside down underwater for a bath. This is bordering on torture.
Misting to provide humid air to breath is the equivalent of you being forced to hold your breathe except for twice a day when someone allows you to inhale some oxygen. You need to breath all the time and so do your hermit crabs.
Land hermit crabs need to be able to submerge in both fresh and ocean water to adjust the salinity of their shell water, replenish shell water, clean their shell out and prepare for molt. Placing them upside down in water once a week is very stressful and makes it impossible for the crab to maintain their shell water correctly.
If you have a small tank simple bowls work perfectly fine for pools. Just change out the water every couple days so it stays fresh.
If you have a bit more room you may want to try bigger pools. Adding airstone bubblers to your pools will help them stay fresh longer.
If you have a really large crabitat you may want to try the filtered waterfalls that I use. I have two Tetra 19525 Decorative Terrarium ReptoFilter in Sandstones installed in my 150 gallon crabitat. They each take two medium size charcoal cartridge filters. The waterfalls sit inside of sturdy Sterilite bins. They are a harder plastic so they are not doubled up. These have been a great addition to my tank. The crabs enjoy the flowing water and the humidity from the pools.
Maintenance is pretty simple. Once a month I use a small wet vac to suck out all the dirty water and gunk. Then I change out the filters and refill. I haven’t had these in place long term but I’m planning for twice a year thorough scrubbing of the filter housing.
Using doubled bowls makes water changes a breeze. Lift out the top bowl and dump, the bottom bowl prevents the sand from collapsing and disturbing possible molters.
Gladware bowls make cheap, easy pools.
Be sure your crabs can easily climb out of the water. Plastic canvas is commonly used to make ramps but you can use fake plants, rocks etc
A Stanley Wet Vac makes quick work of sucking up the water from my pools.
Beware of pool leaks! You can flood your tank so easily. Take precautions to avoid leaks such as doubling up your bowls.
Bubblers and waterfalls splash and can quickly over saturate your substrate and cause a flood.