bettafix is not something you should be adding on a regular basis. I would discontinue it.
It's a really bad idea to use a heater without a thermometer. How do you know it's even working? You should also use an in tank glass thermometer with the heater and make sure it is holding a steady temp between 78-80F. If the temp is jumping around, hotter, or can't reach that high, you need a different one.
Also it's really important during water changes to match the temperature of the new water to the tank water, and you can't without putting a thermometer under the running tap. Because by the time you wait for the tank to heater up, your betta is sitting in the cup cooling off, then the betta has to be warmed back up again.. best case this creates unnecessary extra stress, but you can also shock your betta doing this.