Increasing and Regulating Humidity - Help Needed

[mercury6]

I'm posting here for the first time with questions about increasing and regulating humidity, but here are some details that may help pinpoint my problem and how to correct it:

What is the volume of your main tank?
20 gallons

How many hermit crabs do you currently have? How big are they?
2 large Purple Pinchers

How long have you had the hermit crab(s) that you are concerned about?
about 11 months

What substrate or combination of substrates are you using? How deep is it?
50/50 sand/EE mix, 7 inches at its deepest end and lower by the water dishes

What do you feed your hermit crabs?
variety of fruits, veggies, fish, meats and a crab food mixture

Do you use distilled, bottled, tap or other types of water?
softened, filtered, non-chlorinated well water

If you are using a dechlorinator, what brand is it? Does it state to remove all heavy metals, chlorine and chloramines?
N/A

What brand of sea salt are you using to supply your hermit crabs with salt water?
Instant Ocean

What heat source are you using and where is it positioned?
currently using an UTH placed above the substrate at the back of the tank.

Have you calibrated your thermometer(s) and hygrometer so that you can interpret accurate readings?
am calibrating one while another combination thermometer/hygrometer is in the tank

What is the air temperature range and humidity inside the tank?
right now the temp ranges from 72-76 degrees F. the humidity was pretty stable in the 70-80 range, but since the outdoor temperatures have recently begun to drop, it has been dipping into the 50-60 range unless I constantly add moisture to the tank.

What is the highest temperature that the substrate near the heat source reaches?
72-73 degrees F

Do you have any photos of your tank or hermit crab(s) of concern? If yes, please attach them to your post.
photos not available yet, but i'm working on that. both my PP's have recently molted and are active and seem to be fine, but the humidity is a recent problem. The tank has artificial plants, climbing structures, a hemp net with moss and shells and two pools with bubblers (fresh and saltwater) deep enough for my crabs to completely submerge in. water is added to the substrate each week and still the moss dries out regularly. a week and a half ago i was having problems with mold, now the problem is swinging to the other extreme. i'm not sure what i'm doing wrong except maybe the drop in temperature outside has something to do with it? i'm adding a hood with incandescent bulbs (day/night) to boost the temperature. will the warmer air better hold the humidity or can i expect it to dry out further? i'm ready to pipe my cool air humidifier into the crabitat. sorry, i know humidity is a common problem here, but any advice is greatly appreciated. thanks!

[Wai]

Warmer air will dry out the humidity a bit more. The easiest way is to just spray the air and sides of the tank with water; that will boost up the levels very quickly. Adding air stones into the water dishes and setting up a moss pit may also help a bit. If you can get around to setting up a fogger (without risking the hermit crabs touching or falling into the hot water in which the fogger is submerged), you can boost up the levels too. However, foggers do not seem to last very long and need replacement every few months depending on how often you turn them on.

[mercury6]

Wai, thank you for quick reply. definitely appreciated!