My loaches' favorites are zucchini and cucumber. I would deseed them though b4 you put them in the tank or you'll have a mess on your hands. Your loaches will love snacking on these veggies.
No romaine/lettuces! Another member suggested not to feed broccoli. Why? They will eat some of it but most of it ends up floating around your tank , sticking to filter strainers, and making a mess!