small tank w/ crab

[licerbob]

I am in an Oceanography class at my High school and the year ends in about 3 weeks. (class of 2014!!!)
My teacher is trying to adopt out are "specimens" that we caught in the Chesapeake bay at the beginning of the year. In me and my partners tank we only have a small (less than an inch) mud crab (not sure what specific species sorry I will try to get a pic) and a long pincered hermet crab.

If I adopted these 2 tiny crabs would they be O.K. in a 1 gallon tank w/ under gravel filter and sand? The tank is kind of more short and fat rather than tall and skinny so lots of ground space was my thinking since the crabs do not swim?
I know there brackish and my class tank salinity is on average 28ppt. I THINK 2 teaspoons per gallon should get the salinity up to that? Dose any body know there salinity range?
I have a larger (40 g) essentially freshwater tank (there is a little bit of salt to help the fish but I do not know how much right now because of water changes and stuff) with some Mollies, guppies, platties, and ghost Catfish

If I do need a bigger tank what size? and can any fish go in with them?

Obviously I need some tips please help! Thanks alotl

P.S. this post is also in the fresh and brackish invertebrates but I can't get a response and am running out of time.

 

[dingusplease]

Hi and welcome to AA! Pics of the two crabs in question would definitely help. Honestly, I wouldn't recommend the crabs as an on-the-spot decision. It is good to do considerable research before setting up any SW tank, especially if it's your first. SW can be very difficult and complex if you don't prepare for your tank in advance.

Have you ever done a saltwater tank before? You'll need a tank larger than one gallon, in fact, the larger the tank the better. Temp and parameter swings are a major concern for small tanks.

In addition, you'll need to cycle the tank, which can take weeks as I'm sure you know. Live rock is almost a must, as it will house the BB that filter your tank biologically.

You mention that the crabs are brackish, SG at 1.021, which is borderline full saltwater. In that case, assuming you use a conventional aquarium salt, you will need around 1/2 cup of salt per gallon, and then more and more salt for water changes.