They basically hang out where the return hits the glass, they come down to feed infrequently. When I throw food in the return just pushes it to them and that's how they get most of their food.
500gph in a 90g tank and that's where they sit... I assume they come from really fast moving water.
I've also looked at them a little more closely in the last few days and looking at their pectoral fins they do the same thing that a lot of other river dwellers do. They use em to hold tight to any flat surface. Reminds me of a goonch or a butterfly loach/stingray pleco