White spot on goldfish (picture)

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Picked up two goldfish a week ago today, one black moor and one fantail. The Blackmore came with some of his fin damaged with a sort of hole in the centre but seeing how cramped the tank was in the shop I just thought he'd been injured by another fish.
He was really still and quiet up until 2 days ago where he seems to have cheered up and swims around happily but today he's come up with a white cottony spot inside the gap in his fin, not bothering him but what should I do?
The water is ammonia 0.1, nitrite 0 and nitrate 10 PH 8

I know the tanks too small even though the man in the shop (supposedly a good independent aquatics store....) told me I could easily put 3 in a 21 litre tank for a year before moving them. They are currently less than an inch long and in a 21 litre tank with a live plant, no idea what it is though.
They will be moved to a 100litre in the next few weeks and the fantail will go in to a family members pond with the other goldfish once big enough.

So.. What do I do with the white fluff? I've been doing 25% water changes every day since we got them and adding interpret ammonia remover but I don't feel I've done enough

.

The orange one is a comet and he'll be very happy in a pond :)

The black moor looks like he has cotton wool fungus, but have a google for some images to see what you think (not while you're eating!). If you think it is cotton wool then it's easily treatable. Something like this should work. Keep a close eye on the water quality too, especially if the tank wasn't cycled before adding the fish. Everything looks OK so far with the water but you have to be careful with new tanks.

And go back to the shop and kick the guy in the balls for saying you can have three goldfish in a 21 litre tank! 21 litres is too small for any fish really, but they'll be OK for a few weeks until you can move them.

The shop got that wrong too then is he definitely a comet? They all looked the same to me
Got home a few hours after posting and the white bits disappeared and fishy seems much happier and the hole in his fin doesn't look like its getting any bigger. I've had most animals at some point and these little fish are much harder than all of them to keep healthy it seems, in fact , I think my 3yo ds is easier ???? I can't get the ammonia to go completely even with daily 25% water changes and ammonia remover so it's being tested a few times a day but always stays at 0.1.

It was 3 for £10 on the goldfish so I think he just wanted them gone in one go he didn't tell me what the plant was called either and I seem to be killing it, have put a picture of it up. Its got roots growing but I'm using pebbles so it can't grow in to them. Should I bin it and get a new plant?

Sorry for the questions, I want the fish to stay happy, ds has lost interest so they're mine :D

Ooh, looking at him from the side, he might be a bit fantail. He looks quite streamlined compared to most fancy goldfish but that might just be because he's only a baby, or it's possible that his pedigree isn't 100% fantail (goldfish are like dogs- same species but many different breeds which can be crossed). An adult fantail looks like this and an adult comet looks like this. As you can see, they both have very long fins but the fantail has a more compact body. This means they are unsuited to life in a pond, unfortunately :(

Check out gumtree for bigger second hand tanks. You'll need at least 120 litres to keep these two healthy and happy.