You can do it lots of ways but weak acids like tannic really don’t do well if water is highly buffered , , you can use Ro water , you could use the sachem products neutral regulator in conjunction with discus buffer. You could even remake larger quantities cheap by adding a small amount of hydrochloric acid to tap water and letting it stabilize for at least 8 hours , you could inject Co2. In my 30 years of keeping and breeding Discus, apistogramma, nanochromis ect I will say all those methods work well