Join Date: Feb 2019
Location: Los Alamos, New Mexico
8.6 does seem high. Your betta won't much like it, and your tetras won't like it at all.
You have a number of options for lowering pH.
You can try diluting your tap water with deionized water. If you don't want to set up a reverse osmosis system -- which is a lot of investment for a ten gallon tank -- you can purchase gallon containers at your grocery. You can experiment with how much deionized water you need to mix with your tap water to bring the pH down to a reasonable value, say, 7.5. You don't want to use straight deionized water because it will be too soft. If you're doing 20% water changes every week, the cost should not be unreasonable for a ten gallon tank.
You can also try acidifying the water, depending on how comfortable you are working with chemicals. I live in an area with alkaline water, and I used to neutralize it with hydrochloric acid (I bought a big jug at the local hardware; they tend to label it "muriatic acid"). I had to work out the correct dosing: If you add enough to bring the pH to your target, the next day it'll be alkaline again (though less so) as CO2 slowly diffuses out of the water and raises the pH again. This will not lower general hardness and you may not wish to play with concentrated acid. It worked for me, more or less, but I eventually switched to deionized water and then moved to an area with less alkaline water and took up keeping fish and plants that were okay with it.
If you inject enough CO2, you can bring the pH down. It's not rare for serious planted tank enthusiasts to inject enough CO2 to bring pH down a full point, though most probably don't start at pH 8.5. (Higher pH needs more CO2 to produce a given pH change.)
Old-time discus keepers sometimes filtered their water through peat, claiming this removed calcium and lowered pH. I believe the latter; peat has a lot of humic acids in it. But, while aquarium peat is sold in some fish shops, it's a lot of fuss and the results are uncertain.
If you were planning on CO2 injection anyway, first see how much that nudges down the pH. if I was in your place, I'd probably go with diluting tap water with deionized water as well. It would be helpful to know what your carbonate and general hardness are.