Zebra loaches (bootie striata) would be good for the size tank you have.
Try to
get at least 3 if you have the room. They like to be in groups.

Yoyo loaches (bootie almorhae) would be my second choice, but they would
eventually get too big for a 30..

Clown loaches (bootie macracantha) would be my last choice. They would
definitely outgrow your tank.

If you have shrimps these loaches are not compatible.

--
Thank You

Dominic
http://www.dlink.org/aqua