Feeding herbivores question

I've got a small 1" blue hippo tang and several omnivores. They have not taken to the nori strips or Formula 2. I have little in the way of algae on the rocks for them to pick at. The Formula 2 kind of sucks in that the instructions say to put a frozen cube in the tank, and that's in. I'm currently rubber-banding a nori strip to a small rock that is placed in the sand bed, and also keep one on a clip on the side of the tank. Frankly, it's been a waste of money as they simply ignore the strips (the snails love them).



I know this is a bit of a sophomoric question, but I've not had a group of fish like this that wouldn't attack nori. Any thoughts on promoting the eating of their veggies?