HCA: Hermit Crab Association

I have two very small purple pinchers (about 1 inch or smaller). We've had them about 2 weeks. They have about 3 inches of sand, plenty to fully burrow. I give them fresh salt and drinking water daily, sponge in each bowl. The temp and humidity in the tank hover around 84° and 80%. I noticed today that when I open the lid it smells a bit fishy. Not a rotting smell, just like ocean smell. They are both alive because I've seen them both move. What else could be causing the smell? There's no old food in their bowl either.

I wouldn't worry, mine smells weird sometimes too. also because most people give them fresh fruit/vegetables/meat, the crabs might bury it and they will quickly rot and smell. in my experience, moldy raisins/grapes have always had a strange fishy smell. the sponges may cause a stench as well, because it can get moldy quickly and sometimes can be a breeding ground for bacteria.