I have a planted aquarium and i've aften seen my goldfishes with green poops on their back. i know they eat alots of my plants...

Over-EATING is something that a goldfish doesn't care much so what with all the plants they eat? Also, on top of that if i feed them, wouldn't that cause overfeeding and problems?

It is my understanding that a fish can not hurt himself by eating too many plants, in fact I think it is really good for them. How frequently do you feed them? I would not worry about it, but if you are a little fasting every once and a while doesn't hurt. If you know they eat a lot of plants try just feeding them once a day... however the best thing to do is just watch them and see how their behavior changes when you change their feeding schedule.

I am feeding once a day.... behavior??? kinda hard cause anyone anytime gets near the tank, they will always swim up and expect to be feeded...