Thanks JoshBA. I have learned a few things on this original setup that I won't be doing on the 2nd. First, the wall is WAY too tall. It doesn't need that much water and because of it, the crab is almost never in the sandy side. There needs to be more of a slope so he feels more comfortable climbing in and out of the water. I usually see him perched up on the rocks, but hardly ever in the sand. I've seem some setups where sections of PVC are buried in the sand to create tunnels which I think will work well. The dimensions of the 20 long I'll be using will be better for this too.
I think I will also put a good layer of the lava rock under the sand on the dry side so it can drain.
I've been dropping various clippings of plants in the tank from time to time, and no surprise, hygro sunset seems to be doing just fine in there. Duckweed also seems to have no issue staying green in the brackish water also. Weeds will be weeds I guess.
Switched out to a 20 long. I went ahead and sealed up the 'dry' side. I couldn't figure out how to drain without the sand moving over to the wet side. I'm keeping the wet side sand free because my little tetra sand kept getting stopped up with sand in the impeller no matter how I try to pre-filter it.
Also got my little guy a girlfriend. They've been flirting all evening tonight. He's waving, she's creeping back and forth. Pretty funny to watch.
Finally added some live plants to the dry side. Some 'lewisia' (forground) and some christmas fern (background).
They dig in the mud just to hang out. The female will spend days out of the water. She was carrying a ton of eggs. No clue if they hatched or anything. They hatch out of eggs into a planktonic stage and I'm not sure if they could survive in an aquarium setting. There's no way to protect a planktonic stage baby from filters.