Hi, my goldfish have been in a temporary tank of 980L for 6 months without any issues. We regularly clean the pump. There are 12 gold fish in there of various sizes. I have noticed that at least 1 goldfish has developed a growth on both gills, see photo, and it looks like some of the others are starting to get this as well. Any idea what this is and what treatment is available? Many thanks Alex

Thanks.
Could you possibly take a photo from further away as well, so that I can see this in the context of the fish itself?
I'm thinking that what you may be witnessing is the breeding tubercles on the male fish which tend to be evident at this time of year in particular, and you may notice them chasing the females too.
However, a photo as described would help to definitively work out if it's this or something else.

Also, can you provide liquid-based test results for ammonia, nitrite and nitrates in the water? Thanks.