The hermit crabs I have in my tank came from the gulf. I have maybe 15 in my tank.
We have thousands of them here in all sizes and shells. I went down to local hobby store and bought a basket of shell for like $1.50 and picked out the hermit shells and put them in the tank. Took about 2 minutes before they started moving in.
I also have 2 fish in there I saved from being used as bait. It was either the hook or my tank.
One is about 2 inches and is silver and yellow, looks kinds like a baby Jackfish.
The other is what we have always called an Angelfish. Below is a pic of it. We use it to catch 4 to 7 foot sharks or big Bullreds.
I believe what the Game Warden frounds upon is keeping any type of game fish in an aquarium.
I have numerous friends with sw and fw tanks that have had lots of fish and critters in them that has been caught locally.
I am going shrimping with a friend on his boat soon just to see what I can get from the nets.
Down in Galveston I have seen things along the rock groins ranging from Lion Fish to Anemons believe it or not. You have to catch the water when it is clear which is rare here.
My fw water tank used to have a Big Mouth Bass in it that I caught in a drainage ditch with my cast net. He was an eating machine. He was about 10 inches long.
The only prob I see with you keeping critters is what ntvflgirl said, you could introduce parasites into your tank. Chances are you wont, but remember there is a possiblity.
As someone here has on there signature...."Your tank, Your choice."