I use craft mesh hammocks for mine, pieces of plastic plants, floating plants like riccia.
It could honestly be the filter current, when I upgraded Pancake's filter, he spazzed for a few days.
As for water level, that's actually fine 18" is about the deepest you want it to be, but they come from rivers. Normally, that would mean that they shouldn't have a problem with a current, but petstores tend to keep them in slow moving tanks, so he'll really need to adjust.
Caves don't always help. Mine likes to hang out in the plants. he has a cave under the driftwood, but he's usually pretending to be seaweed.
they don't have to be expensive plants either, just something he could wrap his legs into like that last photo, and just one of those would most likely make a difference. They're a prey animal in their native environment, and you have to think like a prey animal to make him comfortable.