Hmm...if I were you, I would research all the betta diseases and see if any match...my reckless little betta always bumps his scales off and tears his fins so if there are dark spots on my little fish, I don't usually worry. Though he might have a slime coat problem and it may be his slime coat that is deteriorating, and not his scales. I would look for betta food that helps with regrowth and water conditioner.
The pimple like thing could be an ulcer, in which case I would be concerned that this fellow has a definite bacterial infection, or it could possibly be lymphocystis. Judging from the picture though and how the sore is ringed with red, I think we are dealing with an ulcer.
You may continue with the Betta Fix but be aware that this medication acts mainly as an antiseptic and won't necessarily cure anything. Also, be extra careful not to overdose this as it can cause harm to the betta's labyrinth organ. If this medication fails to work, switch to Maracyn Two.
Yes but you'll have to adjust the dosage for whatever size tank he is in. The packets are designed for 10gal.
If your tank is less than 10gal, then follow this:
get a container that can hold 5 cups of dechlorinated water. Add in the first dose (I believe it's two packets). Mix well. Now, the ratio is 1/2 cup of medicated water per gallon of tank water. So if his tank is 2 gallons, add 1 cup of medicated water.
Unfortunately, Maracyn Two does not last so each time you need to dose the tank, you will have to mix up a new batch. It's a wretched waste but the flipside is you could kill your fish using toxic meds. :(
There are two ways to do that. If the tank is under 2 gallons, I recommend water changes each day and redosing the medication to full strength. This, unfortunately, means wasting meds. Although, not if you do the split method like you're using.
If the tank is 2 gallons or over, then I suggest following the instructions on the box as if it was a 10gal but dosing it appropriately.