do you have big piece of rock or something similar in your tank? they like the smooth surface of the rock, and as others suggest, cooler fast flowing water.
If you have a big piece smooth surface rock sticking out of the water, and it is fantastic, the hill stream loach will crawl out of the water, and eat the algae on rock surface.
You don't need to do anything they are grazers and you will see the paths on the glass of where they have been, I love them and can't find them locally but I have had up to 6 in a 150, they don't live very long 3 to 4 years at best but mostly 2 years, they leave behind a really cool skeleton. I would like to have some but 12.95 each plus $45 shipping at AZ Gardens is out of my budget.
They tend to disappear for long periods and are very good at hidding but are cool aquarium loaches. I have never seen them come for food like other loaches but I don't feed zuccinni or other veggies, their habits are very similar to Otocinclus.