bristlenose catfish are very effective cleaning tanks, they do however develop 'bristles', and tough skin as they get older to deter would be predators. another option could be 'cold water suckers' these are usually quiet cheap and are extremely effective at cleaning tanks (when in with other fish you sometimes have to keep an eye on them as when they run out of algae they can turn on the slime coat of sleeping tankmates) they are nice and soft and will go down a treat if a monitor caught it. and despite what their common name implies they can happily get along in water up to 27 degrees, and can adapt to various water conditions. i have had green tree frogs eat them and they arent as tough as a monitor.
also bernd eidenmulers book is a must for keeping monitors in my opinion, great book