I'm not sure exactly whose test kit it is, I think it's API and I also have mardel ammonia testing component.
I don't use the gravel vaccum exactly (i do not like to use them, have never had luck in using them), but I do brush up the stones and get the junk out.
Ok.
Check your tap nitrate reading as I find with goldfish its really rare not to have a nitrate reading even when your tap nitrate is 0, and that is fairly rare too.
It can be as the bacteria from the fish can enters the tank, I've had it with a fish and it passed it onto some others.
Read an article on septicemia and it said septicemia can enter the tank, more though cuts and wounds.
Just water changes on the main tank.
How long has the goldfish tank been set up.
I would get the gravel vac out, and see if that fetches the nitrite reading down.
As the longer you have that nitrite reading the fish will be stressed and get ill.
I did a water change earlier toinght so I'll do another one tomorrow. (Unless you think another one tonight will benefit them??)
The fish have only been in the tank for a week together. But the tank was setup longer before they were in it, I can't remember the exact amount of time though (it was a while, I thought I'd never get to put the fish in.)