I have one of those tanks too! It's THE BEST. I wish I had more room in my house for another but mine is in the garage now and I don't even have room in the garage for a second.
For your goldfish - do they have dark gray circles around their eyes? You should be able to see them from the top when you feed them and they get close to the surface of the water.
Timmy & Chewy & Ike 
Betty & Frick & Frack & Tina & Nyrtle-Sweetpea
Baby Timmy & Ranger