Hi, sorry to hear about your betta
We're really going to need a lot more information from you in order to try to figure out what the problem may be. If you could copy paste the template below and fill in the answers you can, with as much detail as possible, that will give people a place to start
Tank size: 20 gallons
tank age: I've had this tank for 2 years and a half
pH: 7.0
ammonia: (doesnt show)
nitrite: 0
nitrate: 0
kH: 120/80 pretty sure its 80, it was really hard to tell
gH: 180
tank temp: 70 pretty sure
Fish Symptoms (include full description including lesion, color, location, fish behavior): hes standing still for a good while, moves positions every so often, he does tend to eat if he sees any food, his color turned black a little. sometimes i see him top or bottom
Volume and Frequency of water changes: I've done 2 75% water changes this week and also used salt for 2 weeks
Chemical Additives or Media in your tank: none
Tank inhabitants: I have one cory catfish
Recent additions to your tank (living or decoration): none
Exposure to chemicals: none
Digital photo (include if possible):