Feeding fish the same food all the time isn't good for them at all. A variety of foods thwarts off some diseases, makes them healthier and they grow a lot faster.
I'm glad you asked for some ideas.
Depending on the species and their size, you can feed--
flake food, frozen foods, brine shrimp, Daphnia, earthworms, black worms, meal worms, blood worms, mosquito larvae, crickets, freeze dried foods, pellets, spiralina tablets, krill, peas, lettuce, cucumbers, zuchinni.
I've fed all of that to my fish I've had at one time or another.
Romaine lettuce seems to be the most popular variety in the hobby, although I also feed my fish leaf lettuce from my garden. It is often recommended that you blanche the veggies for a few moments before putting them in the tank (pour boiling water over them). This softens them up and brings the flavour out. However, I never bother blanching, and my fish eat anyway. Sometimes you have to offer something new a number of times before your fish recognize that it is a new food. I would guess that your livebearers and your gourami would like some vegetable matter in their diet. Your otos, too. Clip the veggie to the side of the tank with a veggie clip, or wedge it under a rock or piece of wood if you don't have a clip. I find that my livebearers and other small fish really like fresh carrot greens to nibble on. Try different things until you find something they seem to like!
