Parasite Help

[Julixttx]

Recently, about a week ago my brothers goldfish began getting a large red spot on its back. We assumed it was an ammonia burn from adding more fish to the tank so we removed him and put him into my tank with a lot more room. The spot started getting better, but today it started spreading to its other side in smaller dots. The fish stood still for roughly an hour and when I came back from a phone call there was a worm in a ring shape floating at the top of my tank. I took the worm out and I don't have pictures of it, but I don't know what to do. I don't know if it's a parasite that could harm my other goldfish and Dino, what should I do? Do I isolate the fish with the red spots and fully change the water and filter? (I did a 90% water change before getting my goldfish and my brothers goldfish put into my tank. The filter is new from a few days ago) The tank also has 4 snails.
Inside the tank;
2 inch goldfish (the one with the red spots)
1 inch goldfish
1 Dino
4 snails
15g tank

Nothing like this occurred with the Dino, snails, and my goldfish alone.

Thanks for any help!

 

[Delapool]

On the iPhone so pics a little small for my eyes. Is the goldfish new?

What colour was worm? Is fish pooping ok?

Are spots a sore or a bump? Any flashing / raised scales.

If reading correctly, anything red coloured on fish is new?

 

[SherLar]

I dislike using chemicals in my water unless absolutely necessary, but I do use "natural" remedies when water changes and quarantine won't do the trick. For external parasites, I like Microbe-Lift Herbtana. It will take care of most external parasites, but won't help with internal much.