when is the last time you cleaned it out? as with any animal, a tank full of old food and animal waste is gonna smell. if you clean it regularly (once every week or two), there shouldnt be too much of a smell. of course, its always going to smell like a newt tank, but unless you're sniffing the tank, you shouldnt be able to smell it. if you clean it regularly and it still smells, i would suggest looking under tank decorations for hidden waste, making sure there isnt any mold/algae growing on the land portion of the tank, and checking the water parameters to make sure everything is ok.
also, i would bet that your newt is going to become the crabs dinner pretty soon unless you separate them. believe me, crabs can and will catch newts, and if eaten, the newt is probably toxic enough to poison your crab, which would leave you with 2 dead pets. please separate them, even if it means giving one back to the pet store.
