Continuous hermit crab deaths

Question
Ok idk wut is goin on, I have bought 4 crabs total after a matter of about two weeks one died, I returned him and exchanged him for another one week later another one died and I wuz pretty upset. And finally today at the end of the month another of the first ones died. So now I only have two. The one that I bought to replace the first that died and the very first one I bought, and he keeps burying himself so I leave him there for about five days. The temperature AND humidity is almost always optimal. Has two water bowls, one wit petco saltwater glub, and the other is freshwater with the the purifier to make it safe for them. Two sponges. Change their food all the time. Fruit, fish food, hermit crab cakes, hermit food supplement, lettuce, sometimes meats. Places to climb, and hiding spots. Calci sand.

I'm panicking cuz before I have owned crabs and they lasted for two or three years. I heard that it's not good to buy crabs from petsmart cuz the stress from shipping.

Is it my fault that they r dying? Or is it because stress from shipping?

If it's because shipping and handling where should I buy from?
If it seems to be my fault what am i doing wrong?

I want to own about four. Btw I have a ten gallon tank and people say different thins about how many crabs to own in a ten gallon tank.

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Answer
Roman,    First of all this is not YOUR fault. I happen to know their policies. They are supposed to break down their tank and bleach it and everything in it. You have a major parasite problem , but now if the one you have is buried its about to shed so we have two probably to deal with. Tell you the truth you may lose it too. Unfortunately, They are very hard to get off your crab and time consuming. Look closely at it and you will see small black spots get a magnifier if you have to but, they usually can be seen by the naked eye. You know how if you hold their shell and they climb out practically you will see them also you can sex them this way if you can get it to come out far enough behind their last legs there are two black dots on either sides these are gonopores this is your female so don't confuse that with parasite. Your going to have to break your whole tank down (isolate new crab) Clean with 30% bleach water to water solution.Use table salt as an abrasive. Boil everything else that can be boiled for at least 20 minutes. New sand, new sponge only one is necessary for their drinking water the salt water is for a bath not to drink though they do absorb it through their very soft abdomen to obtain their electrolytes and minerals. Where are you in Texas...if you are near the shore and they sell them there you can observe them better and they are usually healthier. (I KNOW) Also grab a piece of coral it looks pretty in the tank and they love to eat it for calcium. One question did you put a rock , wood or something into the tank from outside? This will introduce parasites. Rocks from outside have to be boiled for about 20 to 30 min and wood baked at 400 for 30 mins. Better to buy a piece of driftwood. I have an old fishing net at the back of my tank and they climb all over it.(or heavy twine is good) I have about 30 in a 45 gallon. I am so sorry you are having a problem they are so comical and silly. You are doing a great job feeding you know that the fruit and veggies need to be removed after about 20 minutes or this can draw fruit flies which also may cause this. The only meat you should be feeding them is albacore tuna abit here and there big treat. If your going to purchase from store again find out what day and approximate. time their shipment comes in and take them right from their bags before they are introduced into the enclosure. These guys are delicate but hardy you know what I mean. In fact if they have a competent their petcare manager is almost always there when delivery comes in and should be competent enough to look at these crabs and check for you take my page to her or him. Now, as far as your crabby most parasite medications are either unsafe or irritates invertebrates. The best way is to drown them without drowning your crab. What your going do is make a salt solution works quicker and fill shell to top for about a minute at a time with breaks in between do this at least four times several times a day. As I said it is probably far gone but, I have seen with patience and persistence you can sometimes save them. Don't forget to boil your extra shells also.Make sure there are always lots available to them. They are alot cheaper to order online and the pieces of coral will be to. In your tank about 4 crabs are fine especially if you get some thick twine and hang it on the sides so they have more room. Make sure you clip your lid if you don't have one get a grated top for your tank. Good Luck, Tina