I'm SO confused

My little Coco died and I don't know why. :(
I thought I had everything right, and since she died Charles has molted with no problems. But now Spider is looking pretty bad too. So I am researching tank conditions, and I keep getting confliting info. Everyong seems to agree that these are the important things:
Water (fresh and salt)
Humidity
Temp
Food
Substrate
I think I'm ok in the water department. I dechlorinize the water and I have both salt and fresh water available at all times.
Humidity- Again, I think I'm okay here. I live in FL which is pretty close to their natural enviroment after all. Even with the air on, the humidity stays above 60% in the house and above 70% in the tank.
Temp- our house, and therefore the tank, is always in the 70s. That's fine for them. On the rare occasions that it is cold out, I put the heaters on and their tank stys warm enough.
Food- I have made changes in the food in the past month. Maybe it was too late for Coco, but what do I know? I have switched form commercial products to a diet that more accurately refects what they would eat in the wild. They always have dried shrimp, calcium, cuttlebone, oak and mangrove leaves and branches, dried friut and flowers, seaweed...everything they eat is organic.
Substrate...I use playsand, which I was originally told was fine, and which is very economical considering how much I need. But now there is a debate about playsand and the supposed harmful silca dust it contains, so maybe that's what killed Coco. I thought about just getting sand from the beach, but that is full of mites, cigarette butts, and Lord knows what else. Besides, I think the county brings alot of that in from somewhere to fight erosion. So no beachsand. How about a product called EcoEarth? It seems expensive, and how so you keep it clean? It is easy to sift the sand for little poops and old food, but that stuff is brown, and I have heard that it compresses, and holds moisture and molds. Yuck. Some poeple use extra fine gravel, but it is hard to find and hard to clean and expensive. Besides, gravel is mined, so who knows what it contains. Some people use marine sand (aragonite), again expensive and hard to clean and hard to find. Some people swear tht they have used the playsand for years with no problems. Some people say that when they are moving the playsand in the tank, dust comes up and chokes them. I have never seen or noticed any dust in my tank when I put the sand in or dig through it. I think I'll stick with the sand for now, because all the other crabs look fine. Charles even molted in the tank with no problems last week. They supposedly don't molt if conditions are bad. Spider is due for a molt, and Oprah looked just as bad before she molted, so maybe he is just getting ready to molt.