I don't understand; it's easier to set up a whole new saltwater tank than to switch out your freshwater to a sand substrate and put in an HOB? It's your choice, obviously, but it seems a shame to give up on the loaches. They really are fun fish.
Some of the slimmer loaches are rather eel-like, e.g. kuhlis and other Pangio loaches. They also fulfil your requirements for temperament and color. They do prefer a soft substrate.
You can try, but don't expect much. Fish don't get the sympathy that furry pets do.
What I would do is make it clear to the store owner that I will not shop at a store with such poor standards. Then follow through.
They may not recognize the pellets as food, or they may be holding out for their favorite junk food (shrimp) instead of eating the food that is good for them.
There are a couple of things you can try.
One, crumble the pellets up and offer them at the same time with a small amount of brine...
An inch is plenty; too deep and you will have anaerobic bacteria growing in the gravel. You can use a soil substrate beneath the gravel. You don't need a fancy aquarium soil; a cheapo garden soil such as Hyponex works fine. Do not use potting soil. If you have access to natural topsoil that...
They are an acceptable food, but you will need to supplement with more nutritious foods. Will your fronts eat pellets? A high quality pellet should keep your fronts healthy, with brine shrimp added as a treat.
Still too much of a list.
So, are you set on hermit crabs? I just don't know what kind of herps could live with hermit crabs, but the list of fish that could live with them is quite large.
In this case I would not build up a land area- far too much of the tank would be 'dead space' filled...
Baby brine shrimp are very nutritious. Adult brine shrimp are not very nutritious at all. Ask your supplier if they can get frozen mysis shrimp; your fronts may like them just as well, and they are more nutritious.
How about Congo tetras? They're quite handsome, and are the only fairly large African schoolers I can think of.
Not what you're looking for, but do you think you could get a butterfly fish in there? That would round out your wacky African predator theme.
If you have organic matter beneath the gravel you will get some gas buildup, mostly CO2, from the bacterial decay. I get this in my planted tanks, which have a layer of leaves and soil beneath the gravel.
Brine shrimp are fine, but not as nutritious as the other two foods I mentioned. Hold on to those daphnia; if you get your frontosas to breed, the young will enjoy the daphnia.
I suggest trying to find some live foods. If you can go out into the country and poke through ponds, wet ditches...
What little vinegar residue may be left is harmless. Vinegar will help with calcium (hard water) deposits, but has no effect on salt deposits; for that the best thing is hot water and elbow grease. You'll just have to fill the tank, scrub, and drain it a few times to get all the salt out...
There's no threat of captive axolotls going extinct; they may well be the most common salamander in captivity worldwide. But the captive populations may not be suitable for replenishing the wild stocks due to extensive selective breeding and genetic bottlenecking.