Grey patch on goldfish

[DuzMano]

People should be back from work soon I guess and will be able to shed more light and contribute soon

[DuzMano]

Looking at those new pics I'm not sure about the mmonia burn as I think that is normally very dark, if not black in colour normally... The first pics it looked dark, but these ones it is very much a grey as you mentioned.. So maybe I have you barking up the wrong tree, but the test kit results will still be really important in getting a diagnosis, and a kit is just generally a must have to keep around for such circumstances as these

[DuzMano]

Hindsight is a wonderful thing, but if doing this again I'd have kept the old small gravel, unwashed and 'topped it off' with your larger gravel, and I'd have reused the sponge/media from the old filter somehow into the new to have retained as much of the beneficial bacteria as possible, but as it stands this is all moot. As I say, hopefully some more knowledgeable folk will be along soon enough to help and guide you through this

[fr499y]

Water changes with seachem prime as that converts Ammonia into ammonium? That’s not toxic to fish. As soon as you see any ammonia or nitrite showing do a water change.

Looks bacterial to me but one of the others will pop up at some point and confirm/dismiss lol

[Martinspuddle]

Greetings & welcome to

[plankton]

Welcome to the forum.
It looks to me like there's several things going on here.
The main thing is to get rid of ammonia and get the filter/tank cycled.
I'd suggest water changes with Seachem Prime and Seachem Stability, I'm only suggesting the Seachem products as they will work together, any other "starter" won't work with Prime as you need to leave it 48 hours before adding something else like Tetra Safestart.
The major problem is that cold water cycles much slower than warm water, so you will probably be doing daily, or almost daily water changes for weeks. While it's cycling you need to keep ammonia and nitrite below 0.25 ppm.
When that's done, if the fish hasn't recovered, you can look at an anti-bacterial meds, but be aware that some can upset the filter munchers, especially with new set-ups.

[DuzMano]

Any news on the water parameter test results yet?