A DiY Co2 system is by definition a little inconsistent, so its possible that your formula might be overproducing (or under) for some reason, and increasing (or decreasing) the Co2 you used to have. I've never heard of running walnut in a filter, that's really interesting! Do you have a websites on hand with literature about that? I googled it, but to no avail. Also, Tannins in water is actually Tannic Acid, which will drop your ph, but not to a very large degree.
How are you testing your PH? is it a titration kit? if so, those can be a bit unreliable. Aquarium PH is constantly changing, the only way to get a truly exact measurement is with a probe.
With your plant growth as much as it it, and depending on your substrate and lighting, you might consider some sort of ferts, to balance the iron and other minerals the plants are absorbing. Of course water changes will do this for you as well, so thats up to you. Test your tap water, or where ever you're adding water from, to see if it has a low KH. Ie really soft water. If so, then it could be the water your adding is low. You can fix that with Baking Soda, or if you bake your Baking Soda you can have Sodium Carbonate, which also can fix it. One raises PH, one lowers....i think anyways.
Finally, what kind of lamp are you using?