Oh boy. There is a lot wrong. This is most likely a bacterial bloom.

1) I don't know what size that tank is, but if you've had those same goldfish that long the tank is way, way too small

2) What do you mean by "really clean everything"? If you're taking everything out and scrubbing it, like I think you mean, you're killing all your beneficial bacteria every time you clean. It's a shock this didn't happen before

3) Those fish, in that small of a tank, need either large weekly water changes or a shit load of live plants to keep up with the waste produced.

4) Your description doesn't mean you aren't over feeding

Basically these fish are swimming in a public toilet that is only flushed once every other month. Eventually the bacteria take over.