coolie loach

they are nocturnal and shy. don't expect to see them too much unless you keep them in a really large group and keep the lighting subdued.

how big a tank are they in and what else lives with them?

They just take a while to get settled in. I've got a group of six eel loaches (close relative) and it took them 2 months from the time I bought them for them to come out of hiding on a regular basis. Then I moved and put them in another tank, and it was another month of hiding. But they do come out now, usually right at feeding time when they zip madly around the tank for about five minutes. If you want to observe them, give them the two things they want most: Plenty of friends for company and lots of easy-access hiding places.

I forgot: Put a sponge over your filter intake. If they can fit in, and they can fit into shockingly narrow spaces, you'll likely lose every one.