Betta Fish: Lost my giant betta in 9 days in a cycled tank. What am I missing?

I got Atlas the giant betta at a LFS 12/20 after cycling for 5 weeks w/ Dr. Tim’s ammonia. LFS keeps their bettas in small cubes with nano fish. He was exploring and active the first few days. By day 4 I noticed he was fighting his reflection nonstop and hiding a lot. On Christmas day 5 he was breathing heavy and laying at the bottom of the tank. I dosed Kanaplex but couldn’t move him into a hospital tank until 12/26. His breathing continued to get worse. I was only able to dose API General Cure once after the Kanaplex before he passed this morning. I still can’t figure out what causes him to go into respiratory distress. He never did pinecone. I noticed a few odd marks on him but it’s also hard to tell with his koi patters. Am I missing something that somebody else can pick up on?

Also for what it’s worth, I got 6 endlers from the same LFS months ago and was told they could go in my 20 long with my last betta, who died approx 10 days after introducing them. So I’m wondering if it could be bad conditions there. I don’t plan on going back regardless or ever introducing tank mates other than snails again.

I also threw out the resin driftwood piece and floating log in case it’s chemicals leeching. I want to throw out the whole tank but I just got it and spent a lot of time and money setting it up.

Tank 15 gallon fluval flex 2.0
Tankmates nerite and pest snails
Parameters 0 ammonia, 0 nitrite, 5-10 nitrate checked daily
Temp 77-79F
Fed Fluval bug bites and daphnia until he stopped eating.