I'm losing another one to dropsey. This is my oldest and one of my favorites. I am beginning to wonder if feeding them black worms directly from the fridge - after rinsing them in cold water - might be part of the problem. The only thing in common with this one and the other two is body size. All three bettas were extremely heavy bodied.
The only other thing I can think of is water changes. I do let them float about 30 minutes before starting to slowly mix the water.
I'm careful that way. I keep separate nets, and etc for each tank.
The companion fish looks fine. It's got to be either how I feed the blackworms or somehow when I change the water
I'm losing another one to dropsey. This is my oldest and one of my favorites. I am beginning to wonder if feeding them black worms directly from the fridge - after rinsing them in cold water - might be part of the problem. The only thing in common with this one and the other two is body size. All three bettas were extremely heavy bodied. 
