Molly ‘barrel rolling’ when swimming, please help!

[Ladymolly]

I have had a Dalmatian Molly for 6 months now. He was in a 10 gallon with 2 platys, a bit overstocked, so I set up a 20 gallon for him. I did a fishless cycle for about a month then I put snails in before him. He was doing well for a few weeks until I noticed slight fin rot and started treating him with Melafix. Yesterday morning, he was swimming while doing non stop barrel rolls. I assumed it was swim bladder. A few hours later he was back to swimming regular. I've been doing daily 5g water changes. Today, I woke up to him swimming in barrel rolls again, so I did a 10g water change. If you have any info or ways to help please I want to save him so bad.
My test kit doesn't test for ammonia I am hoping there is none because of all of the water changes.
20g tank
Running for a month and a half
Has heater and filter, runs at 75 degrees F
Dalmatian Molly and 3 snails

5g daily water changes right now.
Using Topfin water conditioner
I use gravel vacuum

I tried to cycle before, but this is my second tank and I am getting the hang of it.
Unfortunately I'm using test strips at the moment.
Doesn't test ammonia
Nitrites 0
Nitrates 0
Ph 7.5

Fed every other day
Variety of flakes, bloodworms, and algae chips

Had him for 6 months
Finrot appeared almost 2 weeks ago, seems better. Swimming barrels appeared yesterday, stopped, then started again today.
Was using API fin and body cure didn't help and turned water pink, switched to Melafix after 2 days.
He was always a healthy fish before
Anything at all will help!!!!

 

[emeraldking]

Could be twirling desease. Or even something with his brains. Also then they can twirl vertically or horizontally. But if it's twirling desease, there's hardly a reasonable cure for that.

 

[Ladymolly]

Could be twirling desease. Or even something with his brains. Also then they can twirl vertically or horizontally. But if it's twirling desease, there's hardly a reasonable cure for that.

If its twirling disease is that fatal? or can it be accommodated to? I don't know anything about twirling disease but ill do some research
Here are a few photos if it helps. This fish is my best friend. Im looking for anything to help him or make him more comfortable.