regarding the saltwater..i use my tank water and add salt (pool salt) to it..
i don't think they'll eat banana peels..some people will bury a banana peel in their growbeds for potassium, i'd only do that if my growbeds had a healthy worm population
get a good fish food as your primary food, not only your fish, but your plants depend on it.. supplement with greens if your fish eat them