Super helpful info…thanks!
I am well schooled in the ways of hermit crabs. I have never seen anyone build a better habitat than I do. I would be a part of those forums, but they all keep their critters in big empty tubs and it makes me sad. Yes, hermit crabs love to climb. King of the mountain is their favorite game, and they get to the top and chirp. It's pretty neat, and no one knows how they make the noise. They don't like sand, and need a soft , dense soil, so they can burrow underground and make a cave. When they molt, they stay underground for up to 8 weeks. I have had crabs live for several years and grow ten times their original size, which is not common for most crab owners. If they don't have a safe place to molt and hide while the new shell hardens, they either die, or rip themselves apart, limb by limb. So yes, it will have to be a very large tank so I have enough space for several crabs to molt at once. The crabs are fond of tearing some stuff apart, seemingly just for entertainment value, but over the years I have discovered several things they won't destroy, golden pathos being one that's easy to grow anywhere.
The two critters I want to keep in the water require very little. You CAN keep them both in a tank without filtration, as we all see betta owners do, but I think that is cruel as well. The ability to breathe air doesn't mean they should get crappy water. Bettas don't prefer open space, and will cram themselves in the corner as much as possible. I had one that forced me to remove any rough surfaced rocks because he would squeeze himself between them. So I plan to have the water area the about 1/3 of the tank. Apple snails can breathe above and below the water, and will travel about on land. This is my main reason for wanting to combine the tanks. I want to see what these guys do on land. In the water, they are the most fascinating thing I have ever watched in a tank. I had one that would climb to the top of the tank and leap off in to the water, opening himself up and gliding to the bottom like a ray. Another would float himself to the top of the water, waiting until the current carried him to the filter outlet, and he would open himself and glide to the bottom. He would immediately close himself and float back up, doing it over and again for an hour or so. Their shells are fairly boring, but the snails themselves are pretty spectacular. Most importantly, the only reproduce sexually, and I have never had a mating pair succeed in laying fertilized eggs, though I have waited patiently for many a clutch to hatch.
You are completely right about not putting the filtration under ground, and there is no reason to. As long as I protect the tubing, anything going through the land area can be stuff for the crabs to climb on.