i have a twenty long that i keep my two red clawed crabs in. the substrate is sand. the water level is a little over half . it's filled with lava rockes. you can go to home depot and get barbeque lava rocks, twenty lbs for like five$ just rinse them off and then you can stack them and make caves. the crabs really like the lava rock it's easy to climb on and lava rocks are a good breeding place for beneficial bacteria to crabs. i also have driftwood in mine that protrude above the water line. my crabs spend most the time on this wood and most their time above water. they need lots of places to hide as they are territorial and sometimes will fight amongst each other. they are also very dirty so water changes should be done weekly. on my twenty i have two ten gallon filters one on each end so i have good current at both ends of the tank. red claws come from mangroves so they really like wood so wood to climb on and hide on under in is a must. Also they like plants but will tear up or eat alot. i have crypt bronze wenditti in mine and they dont touch it i also have some java fern that is sturdy and the crabs leave it alone. the water is also brackish as the salt in the water helps prevent disease in the water. crabs are very vulnerable after they molt and the salt is beneficiary in not only preventing disease but it draws the moisture out of their shells and it hardens more quickly lowering the stress levels of your crabs. the only other thing in the tank with them is an amano and a rudolph shrimp. they clean up a little and are compatible witht the brackish water parameters. i also feed them peas, shrimp pellets and snails from my other thank. i hope this helps. i also keep the temp around 79 and try to keep the air humid by keeping the top closed. i also have a submersible heater which is a must if you have a partially filled tank. good luck.
