3 weeks ago, I purchased 6 crabs. 3 seemed to be fine, crawling around and eating and such, while 3 didn't move after I put them in the tank and they went off to hide. I left everyone alone for a week, and the 3 still seemed to be okay, while the other ones were starting to stink. One had crawled out of his shell and died, another started dripping brown liquid, and another smelled awfully fishy. I thought it was just PPS, but a mortality rate of 50% within a week seemed high.
Since it had only been a week, I "exchanged" them and picked up 3 more. When I got back, another of the original group wasn't moving, but I decided to leave him alone. I put the new 3 in a smaller tank, so I could slowly increase the temp. and humidity from what it was in the store.
One of the new ones died, and the old one that wasn't moving did too!
So at this point (last week) I introduced the 2 surviving new crabs to the 2 surviving old crabs and they all seemed fine. They were all eating and drinking and moving around a lot. This was all good and fine for a week, but this morning one was just sitting there, not moving at all. I let him be until just now, I picked him up and a leg fell off

Please help!! I have no idea if I am just unlucky or if I am doing a terrible job at crab keeping! Out of the 9 crabs I've ever had in the past few weeks, 6 have died; is this unusually high or what??
What is the volume of your main tank?
50 gallons.
How many hermit crabs do you currently have? How big are they?
4, probably just 3 now, one is very likely dead. All pretty medium, the biggest one is golf ball sized.
How long have you had the hermit crab(s) that you are concerned about?
2 to 3 weeks.
What substrate or combination of substrates are you using? How deep is it?
Sand, 3-4 inches.
What do you feed your hermit crabs?
a dry food combination of bloodworms, cuttlebones, oatmeal, rice. Different fresh food every day, seem to like grapes, applesauce, lettuce, spinach.
Do you use distilled, bottled, tap or other types of water?
Tap, but dechlorinated.
If you are using a dechlorinator, what brand is it? Does it state to remove all heavy metals, chlorine and chloramines?
TetraAqua's Aqua Safe
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?
None right now, I live in Texas and don't often run the air unless I have to, I keep my room around 75. Will probably get a heat source when the weather gets cold.
Have you calibrated your thermometer(s) and hygrometer so that you can interpret accurate readings?
No
What is the air temperature range and humidity inside the tank?
76 to 78 for both.