bacterial bloom

[Hermitsandplants]

I took the water dish out and the sand was almost black and stunk like a rotting pumpkin. i dont know how far out the bacterial bloom gos but i know its at least that corner and am probably going to change all the sand. its a 25gallon so im putting 50lbs of new sand in, so thankfully its not a huge project. but the problem is, i have two crabs underground right now and i am unsure if they are molting. what is the best way to remove sand without harming them? thanks for any input