I'm brand new here, so apologies if I've posted this in the wrong place.
I have a fantail goldfish who will turn 2 years old in a few weeks. He is by himself in a 40L tank (I'm aware this is too small and am working on getting him a bigger one). I've recently noticed some white 'thickening' along the edge of his tail and pectoral fin, almost like little bumps. I've attached photos for reference (which were very difficult to get). The 'thickening' is ONLY along the edge of his fins, and not in the main flesh of his fins, so to speak.
He measures approximately 13cm long (from his nose to roughly the end of his tail) and has been showing no abnormal behaviour. He eats twice a day (pellets and peas with the skins removed), defecates regularly, is not flashing, gasping, or darting, has no issues with buoyancy, and is not lethargic. His scales are shiny and have no marks on them, his fins are intact and not frayed, his eyes are clear, and he doesn't move his gills excessively. He is also quite bold and will swim to the front of the tank whenever I walk past.
I test his water using the API water test kit every two days, and the results are always consistent:
Ammonia - 0 Nitrites - 0 Nitrates - 10 - 20
He has moss balls, a good filter, silk plants that he can't injure himself on, and a wall mounted air stone. I use Bioactive Tap Safe to treat the water and vacuum the gravel in the tank once a week.
I also worry about him incessantly.
Since noticing this 'thickening', I've watched him like a hawk to see whether it spreads. While it has spread down the edge of his tail slightly, it hasn't spread to his body (so I'm fairly certain it isn't ich) or out into the slightly more translucent part of his fins.
Is someone able to tell me what's causing this? I'm ready and willing to take any action necessary to preserve his health if it's something I need to treat.
Apologies for delayed reply (and in advance for my brevity / struggling due to ill-health). Difficulty seeing photo - any further ones would be helpful. Are the bumps white like breeding tubercles (commonly found on pectoral fins)? (Do an internet search for what these look like.) Ich is like grains of salt and all over the body, so unlikely, esp if hasn't spread. Nothing else sounds untoward, so wouldn't medicate. More frequent and larger water changes, or sometimes the occasional dose of aquarium salt, can resolve a problem of unknown origin.