i have red clawed crabs and with any type of crab think of its environment on where they come from... now the red clawed crabs do need warm water i believe around 23'C to 29'C is where i keep mine and they are doing just great...
you can keep them with fish, but do need an area to come out and dry, also this dry area is for them when they molt.. without it they will drown during that process...
the only thing with keeping them with fish is that if they are able to catch them then they will and will eat them or be eaten by bigger fish.. also there are these things that you can put in your aquarium where its like an air bubble under the water.. it stays at the bottom of the tank and with a pump the inside will stay dry and then that is their dry spot.. i haven't really looked into that so i'm not to sure about it..
with the salt use marine salt and i put about a tablespoon (not rounded) per gallon... to make the brackish water.. now with that you have to have fish that are fine with some salt and wont die with it...if you are putting them with your fish
i'm not a super expert with the crabs because I've only had them for awhile, but they are very social.. well mine are and i usually handle them once a day.. and i have mine in a separate tank .. not with my community fish because i don't want them eating or hurt
hope that helped xD
not sure where you got some of theses ideas form. but they are not connected with REDCLAW crabs.
first off, these crabs cant "drown" ,they have gills. and, though I'm sure there can be variation, they cast in the water.
these crabs don't hunt, nor eat fish. unless they are dead. (err the fish , lol)
you only keep Brackish fish, in Brackish water. its and "old wives tale" that "freshwater" fish need/benefit from salt. (its a great medication, used properly, though)
even a 1ppt, only stock with Brackish fish.
finally, the dry area, if its correct, is where these critters live. water is used "occasionally". even feeding should be done at the "shore line".
they will also sleep there. if the crabitat is correct and supply's damp sand for the purpose.
its common misconception that these guy are "aquatic", but need a little land. truth is they live on land and need a little water.
actually, OP. you need 2/3 land 1/3 water.
remember crabs climb. so you can only half fill any tank you use. (you can see now why keeping fish, even in a "nominally, 50g tank, is virtually impossible. 50g half filled =25g. 2/3 filled for land. gives you under 10g of water).
i too though i had "cracked it"(@BerryAttack). time and experience proved me wrong.
I also though of the "underwater dry area". its not very practical. and would require major modifications to the tank. its "do able". that said, the supply of air is no problem. though maintaining a consistent pressure (of air) means you need to go to commercial air pumps.
ultimatly, though, its not what the crabs need. they need a large area of dry land. this land also needs heated, making the ambient temp, slightly above that of the body of water.