Omnivore means that my fish eat plants and ???

I know that's a dumb question but I'm new to the aquarium life . I have two very large Oscars and two beautiful Knifefish.. I know they eat bloodworms, feeders, etc. but what can I feed them in the way of plant material?? Romaine lettuce? Any other suggestions?? I have a 55 gal. tank, and am getting ready to finish setting up a 125 gal. for the Knives.. Thanks for any and all responses...

wetmanNY: Thankyou!!!

What a wonderful site you sent me to! There is so much for me to read and study, BUT, it was just what I was looking for..