I have 8 feeder/common/comet goldfish in my tank. its a 10g. the fish are about 2-3 inches. Is my tank overcrowded? They are not in their adult size yet. I have a Hang on Filter. All i have in my tank is colored rocks and fish and that's it. Will a 40 or 55g tank work with 8 feeders? Please help me out. I also do water changes every 2 weeks. Because they poop to much at a time. i fed them tetrafin goldfish flakes every morning and at night. Can you give me some tips through what I've gone through. 
   

Oh, and you should vary the diet of your fish. Give them some bloodworms, some veggies on top of the flakes.

They are right, you need a much bigger tank.  They need 10g per fish.  Those little fish grow quite fast in my experience and can get around a foot in length or more when full grown.  I'm not sure where you are located at but Craigslist is a great place to look for a used tank at a good price.  A 75g would work, I know that's what you find around my area, they don't usually carry 80g. 

There is a sticky (in green writing) on the page when you clicked on General Golfish Discussion, it talks about feeding veggies and other foods that some of the people here recommend if you want to check that out, its worth it. 

yup.. they are overcrowded, i have 2 bubbles eyes in one 30g
if you don't upgrade soon you'r gonna lose some! they are not feeder fish, whats a feeder fish? is that a fish?
they are goldfish and they need room to swim and be messy.