Betta Fish: Please help. Fin rot? Biting? Tear?

Hi all, after losing a Betta yesterday mysteriously and suddenly in a hospital tank yesterday, my partner bought me a new one. I cleaned the whole big tank out, before putting the new one in. Did a 100% water change dosed with Prime and Stability. When I put the fish in, his fins were pretty pristine with some slight curling at the bottom and top (in blue) Organism Fawn Art Lawn ornament Feather

Over the course of today though, he now has a tear at the red circle, and I think additional curling.

Insect Pollinator Arthropod Sky Plant

Is this fin rot? And if so, how do I treat it?

There were a couple of green neon tetras in with him, which I've since removed just in case they're nipping his fins, or he's nipping from stress. There doesn't seem to be any real hard edges in the tank also. Photo of tank and water test below. Any help would be greatly appreciated 🙏🏼 I really don't want to lose another fish!

Paint Cylinder Drinkware Cup Font
Plant Leaf Botany Vegetation Grass

Housing:
How many gallons is your tank? 8 gallons
Does it have a filter? Yes
Does it have a heater? No
What temperature is your tank? 26.4C
Does your tank have an air stone or other type of aeration? No
Does your Betta have tank mates? What kind? 4 x cherry shrimp, 5 x green neon tetras

Food:
What food brand do you use? Hikari
Do you feed flakes or pellets? Pellets
Freeze-dried? No
How often do you feed your Betta? How much? Twice daily, 3-4 pellets

Maintenance:
Before your Betta became ill how often did you perform a water change? Brand new tank with fresh, cycled water.
What percentage of water did you change? 100%
What is the source of your water? Tap
Do you vacuum the substrate or just dip out water? New substrate.
What additives do you use other than conditioner? What brand of conditioner? SeaChem Prime, SeaChem Stability, SeaChem Stress Guard

Water Parameters:
See attached photo

Symptoms and Treatment:
When did you first notice the symptoms? Curling worsened overnight. Tear appeared in last hour
How has your Betta’s appearance changed? He appears fine, but he's a new fish so unsure of normal personality
How has your Betta’s behavior changed?
Is your Betta still eating? Yes
Have you started treating your Betta? If so, how? Not yet
Does your Betta have any history of being ill? Unsure
How long have you owned your Betta? 1 day
Was he or she ill or suffering some sort of damage when purchased? I don't think so