Betta Fish: 2 betta deaths in two months

I have a 20g long that currently has 4 cherry shrimps. The first betta I got, a white opal, did great for about 2 weeks then died. I waited a few more weeks and tried again, this time a black orchid betta. Little guy wouldn't eat the Bug Bytes or New Life Spectrum food, only frozen brine shrimp. However, after 2 weeks he stopped eating altogether and is now practically dead. I'm honestly at a loss. Parameters are 0 ammonia, 0 nitrites, 5 or less nitrates. PH 7.4, GH 7 KH 5. Heater set to 80. I only feed once a day and fast one day a week. Weekly 20% water changes with Seachem Prime and Stress Guard, and one squirt of Thrive fertilizer. Lights are set on timer for 8 hours. It's so disheartening that people keep these poor bettas alive in a freaking vase and I can't keep one alive when I'm trying to do everything right 😭