Left fin not working, left side red/inflamed

Tank
20 gallon long, running for at least 3 years
Filter: Seachem Tidal 35
Heater: 100W Aqueon, water temp kept at 80 degrees farenheit
Tankmates: 2 platys (the betta's tank mates when I adopted him; all were in a 5 gallon), and 14 assassin snails (they breed). I am in the process of setting up a Fluval Flex 32.5 gallon to move the platys and snails into.

Maintenance
Change 20% of the water once a week.
Water is treated with Nutrafin Betta plus
I vacuum the substrate - there is alot of algae

*Parameters - Very Important
Tank is well established (was cycled years ago)
Water is tested with strips
Ammonia: 0
Nitrite: 0
Nitrate: 0
pH: around 7.5 to 8.0
Note: Used to use half well water and half city water (got from my parents' home). Well water was tested and I was told it was fine for the fish. Hardness is around 16. I am now slowly increasing the ratio of well water (parents passed away - no longer have easy access to city water).

Feeding
Fish are fed twice a day. Betta was fed Betta bug bites by previous owner so continued with this (about 5-8 bits) along with one pellet of Hikari Betta bio-gold each time.
He also gets one or two frozen bloodworms once or twice a week. Platys get Hikari Guppy food - the betta goes after this too. Assassin snails get Hikari sinking pellets and frozen blood worms. Have to put a tube around each one when I feed otherwise betta will eat all of their food too.

Illness & Symptoms
I have had this fish for 4 months. Fish was is perfect condition when I adopted him and remained in good condition until about a week ago.
Started with him looking like the left side around his stomach was a bit bulged compared to the other side, bent spine, falling to his side, having trouble swimming normally. Appetite remained good. Is able to swim to get food and air.
A couple of days ago, noticed that his left pectoral fin is out from his side and fanned out but is not moving. All work done by his right fin.
Today, his left side around the non-working fin and his gill are red and inflamed looking.

Started treatment about 4 days ago by starting to fast him, thinking that it might be swim bladder disease caused by overeating (thought he got into the snails' food) or swallowing air (when he goes after the platys' food).
Stopped fast a day and a half ago when noticed that his left pectoral fin was not moving. Wondered if he had trouble swimming because of that fin. Checked water paramenters - pH higher than usual. Added a couple of catalpa leaves and a ZooMed betta log.

Behaviour: Activity level and alertness remains the same when I approach the tank, but he now spends time hiding in ornaments and lying on his left side. Appetite is normal.

Explain your emergency situation in detail.
A week ago, I noticed that my betta was having trouble swimming. I thought he might have eaten too much because his left side looked a bit bloated, so I started him on a fast. Two days in, I noticed that his left pectoral fin stayed open and away from his side, and was not moving, while the fin on his right side was doing all of the work. So I stopped the fast yesterday. To my horror, I have just noticed this morning that now his left side around that fin and around his gills looks red and inflamed. What could this be? What can I do for him? Do bettas get tumours? Could the inflammation be caused by infection from lying on that side on the floor of the tank?

 

Bettas do get tunors, actually fairly common. Try frequent water changes and some aquarium salt, may help clear up some of that red and inflammation.

 

An infected injury maybe? Are there any sharp objects in your tank that he could have hurt himself on?

 

Thanks. Will the aquarium salt be harmful to my assassin snails, platys? Would it be best if I move him to a separate, smaller tank?
I try my best to have only smooth ornaments in the aquarium; however, there is one plastic ornament that has a small spikey fake plant attached? I will remove it.

 

Bettas are often inbred for their colours/long fins do a tumor is a real possibility.
Unfortunately we won't be able to tell for sure without pictures.

 

Here are some pictures. I have put him in a hospital tank with aquarium salt but he seems to be doing worse and worse. He has fin rot so not sure if he is not swimming properly due to fin rot, fatigue due to his body trying to deal with the fin rot, his incapacitated pectoral fin, swim bladder issue, or all or a combination of the above (or something else?).