Caveat: I have not kept this fish.
High fin sharks are suckers (catostomids), and like other members of this family should be fed relatively meaty sinking foods (shrimp pellets, nightcrawlers, bloodworms, seafood, commercial fish chow, etc.). Algae wafers and vegetables are fine as a small part of their diet, but they are adapted to eating benthic insects and shellfish, not plants and algae, and their diet should reflect this.
I have never heard of these fish sucking the slime coats of other fish. This may just be a case of confusion with other sucker-mouthed fish, like Chinese algae eaters and some plecos, that are reputed to do this.
I have kept a few other species of catostomids. In my experience, young fish are much more adaptable. Older fish get really stressed out when transferred to a new home and remain skittish indefinitely. Older fish also seem to be more prone to infections.
In my experience, catostomids are not territorial or aggressive, or even very assertive. You may have to take precautions to be sure they get enough food if they are kept with greedy fish like koi. One method is to distract the koi with floating food, then use a tube to deliver sinking food to another part of the tank/pond.
and a whole pond shot would be awesomerr
