Hey everyone, I'm a newbie on here in need of advise from everyone.

Il start from the begging, my girlfriend wanted a fish tank and decided on a biorb. I bought her a biorb 60 in which she wants to keep fancy goldfish. When I bought it I didn't research but since buying I realise how much most fish keepers hate them. In a few weeks (4 max) we will be buying a real tank probably about 200 liters.

Our first fish was a 1 inch (including tail) oranda then about 3 weeks later we bought another one the same size. They were fine together both looking really healthy with dorsal fins standing up. Then the other day we bought a ryukin the same size as the other fish. He looked really healthy when we got him and was fine in my tank for about 3 days then he got some white spots on his fins and looked like he had fin rot on his tail so my local fish shop recommended fin rot medicine which I used then over the last few days he has nothing wrong with fins but it's darting around everywhere but when it stops looks like it raises to the top, some times rests on the magnet cleaner at top of tank, sometimes scratches itself on rocks and has pinky colour appearing on the back of it's stomach. I test my water couple of times a week but foolishly didn't have an ammonia test. So took sample to the shop, the staff said the ammonia was high so do a 30% water change every other day and use prime when doing water changes. He also said that the pink on the fish that is appearing will be ammonia burn which is why the fish is darting around and scratching itself. The other 2 fish seem fine and healthy.

My questions are:
-does it sound like ammonia burns because iv read burns are normally black
-how often should I do water changes and how much?
-what else should I do

Your help would be much appreciated.