Buy a nitrate test kit, liquid, not strips.
Feed pellets, they fill them up better and are cleaner. I only feed and recommend New Life Spectrum, specifically the Thera+A formula. The 3mm (Thera+A Large) should be perfect for them.
Increase the size of the water changes a little each time until you get to about 50% weekly. For now increase the frequency to once every 1-3 days. This will help get the extra nutrients out and starve the algae.
Bristlenose plecos are a great option, trapdoor snails would be good too.
If you are interested PM me for links to a couple articles on goldfish that I wrote.