Does anyone know what this is?

Hello,
I've had Molly's, betas, guppies, angel fish & numerous other types of fresh water fish. However, I haven't had goldfish since I was a kid and it was primarily my mom who took care of the tank & fish. With that being said my daughter won 2 gold fish from the fair last year. I have them in a 20gal aquarium at the moment & in the process of setting up a 55gal. There is the 2 gold fish and clown pleco that was acquired from another kid. The place was in isolation for a month before being placed with the GF. Well about two months ago I noticed this on one of the GF and I have used metholine blue, salt, melafix, erythromycin?, lifeguard & currently para guard... does anyone know what this is and how to treat it? I'm at a loss. Tank gets 1/4 change weekly, gravel clean every week, and full change every 6 weeks (normally monthly but life happens with little kids & sick parents). No live plants yet thats going to be in the upgrade after Christmas. Temp is normally 68 but does fluctuate between 65 & 70 depending on what I have the air/heat set on. So temp is good, tank is testing good, fish are eating & pooping good.

Need more pictures from different angles and showing the entire fish.

How long have you had the fish?
How long has the fish had the red lump (is it a lump)?

You would be better off doing a 75% water change and gravel cleaning the substrate each week and not doing a complete water change each month.

Have you used all these medications?
methylene blue, salt, melafix, erythromycin?, lifeguard & currently para guard
If yes, over what time frame?

Do you have carbon in the filter?
Was it removed before you treated the tank?

What sort of filter is on the tank?
How often and how do you clean it?

Carbon filter, aqua tech 20 gal filter, cleaned every 2 weeks. All meds have been used over roughly 2 months. With a week to 2 week use or what the directed use was. Had fish for 13 months. Lump started roughly 2 months ago but more serious within last month. I thought originally a scale fell off but it was whatever that is that was forming. I don't know if it's a lump but thats what I'm going with.

Wouldn't a 75% change weekly stress them & keep them stressed?
No other fish has this issue

Is the lump semi-transparent and have red lines in it?
If yes, it's most likely a cyst.
If no, then it's a tumour.

Sometimes you can tie a piece of string around the tumour where it meets the body and it cuts off the blood supply to the growth. However, the tumour usually grows back.

Normally you just let the fish be and when the growth gets too big or the fish has trouble swimming, you euthanise.

The fish are normally fine with a 75% water change each week. Just make sure any new water is free of chlorine/ chloramine before it's added to the tank.