I agree with fantailer that you have to separate your goldfish from the other fish.
I disagree to a certain amount
saying that it is hard to breed goldies.
Providing your waterquality is excellent ( and there is a female and a male
lol ) they will spawn pretty easy. Mine spawn every 3-4 weeks during winter and spring when temps are bit cooler. Summer and autum is too hot.
What you need is floating plants like Elodea. After spawning and the eggs attached to the floating plant you can remove it into a smaller tank with water from the parents tank. The ferilized eggs will develop and the fry hatches after 3-4 days ( average ) depending on water temperature.
At this stage please no filtration as the current in the water will be too strong.
The fry feeds on infusoria and on microworms.
The gunk you can suck up with a turkeybaster, but make sure you don't suck up some fry with it.