there's not much that goldfish won't eat! But you could try Anubias (there are many species) and java fern, possibly also java moss. Anubias is a slow grower with hard, thick leaves so most fish, even plecos, leave them alone. Java fern leaves have a gritty feeling and I would imagine are hard to bite as well as unpalatable. Don't know too much about java moss, as I've never had it myself, but maybe someone could tell you yay or nay. It was just a thought.