The Betta has always been healthy and only got sick this once. It seemed to start one day after a water change, not sure if coincidence or not.
^^^^ He got stressed from the KH difference. ^^^^
The general idea is this: the rock in your tank has carbonate CO3 "locked" up with minerals - calcium and magnesium (and maybe other ones). If you drop the pH those acids deteriorate the rock -- to see this drop some muriatic acid (sold at a pool store) on a fresh stone and it will bubble like crazy -- USE GLOVES.
As it deteriorates the rock, the carbonates are released into your water - this directly affects your KH (which is just a measure of these guys).
The amount (concentration) of these carbonates in solution is what fish need TIME to adapt to otherwise they get sick. It was dropped to quick from the water change. When I did this the first time I felt like such an idiot, but it happens and it sucks.
At some point, your KH will stop going up - the equilibrium point. As the nitrogen cycle occurs and acids are released into the water, the carbonates will be used up and the rocks will respond by rebuffering your system - a perfect balance. Same with almond leaf and the peat from your substrate.
Try to remineralize your water to the KH of your tank from now on.
You have options: Seachem equilibrium ONLY does GH. I use a combination of gypsum + epsom salts though for a personal brew.
You need to increase your kH.
Potassium Bicarbonate used for brewing could work - I use this (I like that it gives my plants potassium too) ... if your concentration of potassium is too high, you could run into other deficiencies though, be careful. -- Maybe cut it with the sodium bicarbonate.
Sodium Bicarbonate baking soda will work, but I do not advise it -- the build of sodium ions may mess with something else and the uptake of another nutrient from your plants -- this would require some more research, however.
Calcium bicarbonate - chalk - won't work - it won't dissolve
Magnesium carbonate - WILL NOT WORK
That's about it.
Side note: I think the reason your plants look better when lowering your pH is because you are increasing the concentration of CO2 that is in solution (which is in equilibrium with the carbonates). Yes, air will give you about 3 ppm via Henry's law, but the moment those CO2 pop in solution, they will immediately react with your free hydroxides giving you bicarbonate.