Do Tannin Filters Raise the pH of Water?

Yes, a tannin filter will increase the pH of the water being treated. Tannins are a group of molecules that create an acidic condition when they are dissolved in water. Lignin are a related group of molecules that behave the same way, chemically-speaking.

A tannin filter is more accurately described as an "organics filter" and it will remove both tannins and lignins. Since both of these substances make water more acidic, when you remove them from the water you also remove this effect. This effect will be more pronounced when the water has a low Total Dissolved Solids (TDS) value.

So water that is treated with a tannin filter will have a higher pH than the raw, untreated water. The magnitude of this effect will typically be proportional to the concentration of tannin+lignin in the water to begin with. If you have a lot of it, and you remove it all, the pH can be increased quite a lot.

It's impossible to avoid this pH boosting effect. You can't remove tannins and lignins from the water without having a corresponding increase in the pH.