sorry, didn't mean to scare you! I'm new too. I had never had a fish tank, and bought 3 red claws. for their first month they lived in a chip and dip bowl I had, that held maybe 1.5 gallons of water. I used a water hyacinth as the floating platform. one escaped and i never found him, so I decided I needed a tank... that was 2 months ago.
I went to an auction and got a 20gallon tank for 10 bucks. Then I headed to the LFS and got a hang on the back filter, and I hated it. I went back to the store and got a canister filter, and a large, nice piece of driftwood. I used the filter return to create a waterfall down the driftwood into the tank.
I had a tank with a few garbage plants, the giant driftwood, and WAY TOO MANY ROCKS. I realized later on that I only had about 8-9 gallons of water, with it 3/4 full... and I had overstocked the tank by this time... lesson learned: go slow
back to your crabs... you don't have to go crazy if you are worried about cost. I don't know exactly how much mangroves cost, it was an idea to get you started. they are pretty simple little trees, that are typical in areas where the red claws live. they also grow well in brackish conditions, this is why I suggested them. Not a lot of plants are easy to grow in brackish water. if you search the planted forums for brackish plants, there are many good links and discussions. many plants are cheap and easy to grow. duckweed for example, is a really nice looking floating plant that's really tiny, and duplicates overnight... you can start a tank with 1 piece and have it covered in weeks. the crabs love it. they nibble on the roots, hide in it. the stuff is usually free if you buy some other plants. java moss is great too. I've never paid for java moss, but have managed to collect enough to make a golf ball sized wad with. both java moss and duckweed will help to filter your water. I got my terrestrial moss in my backyard. it looks great and the crabs eat it... but it came with bugs. I've trained my fish to eat the bugs, and the crab does too, but not enough. you can prepare your plants properly to avoid this. I didn't.
if I were you, I'd plan out a landscape... choosing the largest tank you can, just because it's more fun... but a 20 long would be way better than a 10... a 30 long gives you a nicer height for a land/water setup. you are going to want either a submergable filter or a canister... a hang on the back won't be great in a half filled tank. you can make your land by simply sloping the gravel up, piling stones, or with driftwood, or you can go nuts and fabricate something with plastic or foam... the freshwater DIY section and vivarium sections have some nice examples here. it's not particularly important which way you go, all you need to focus on is giving crabs plenty of space to explore, seperate, private hiding places, and easy entry and exit points from the water. plants are a bonus. they like them, they need some plant matter in their diet... and plants in general are good for a tank... but you don't have to start with them if you aren't ready. they really like to move around, and they like to hide in tight tight places. in my tank, the favorite hideouts are a half inch revine that is 6" deep, and a hole in the driftwood that you couldn't fit a golf ball into, but is about 2-3" deep.
giving them a good amount of land is also good for them in terms of getting to know you. mine will now eat worm off my finger... because it is used to being on land near me. mine has stopped trying to escape, I think it likes home... but they are really known to be escape artists, so you should make sure the tank is either covered, or has no way to climb out.
I am currently setting up a 15 tall for my crab, I'm down to one... it's a craig's list tank, and an experiment. I will eventually have a larger paludarium, not sure if it will be brackish or freshwater... my 15 is going to have a land mass in one corner, about 2/3 up the tank, that has moss and plants. I'm going to build the structure out of plexi and driftwood, for the most part. the island will be about the size of a slice of cantelope, or a little larger, and I plan some vines that connect to another piece of driftwood, hanging on the back wall. this should give me about 9 gallons of water to play with, for some other fish, maybe another crab.
here's some pictures of what my 20 looked like when I first set it up.
http://www.aquariacentral.com/forums/showthread.php?t=110466
I now have 7 cherry barbs, 3 otos, a few amano and cherry shrimp, and a lot more plants (and algae.) the tank is overstocked, and I am looking to remove at least the otos. they don't like the crab. I have also raised the driftwood and water line by about 4" since these photos, so it's now only about 5" empty at the top. I want to rescape it, but am starting with the 15 to give the crab a good home during the rescape process.
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