Nigel, You have raised one heck of a lot of good points here and I'm afraid that I am not 100% certain on the answers, but I will try my best. The first thing to remember that pH is a logarithmic scale to base 10 and is about the number of free H+ ions available in the water. The other side of this coin is the number of OH- ions free in the water. When these are in equal quantities, all the H+ are (theoretically) combined with all the OH- to form H2O - 'pure' water. An imbalance make the water either acid (more H+) or alkaline (more OH-). The logarithmic scale takes over to mean that pH 8.0 has 10 times the amount of 0H- ions as pH 7.0, pH 9.0 is 100 times more, and pH 10.0 is 1000 times more. This means that a pH of 7.5 has 10 to the power (7.5 minus 7.0) = 10^0.5 = 3.162 times more OH- ions. To go from 7.5 to 8.0 requires another 3.162 times the amount already there (inclusive of those just added). Still with me? The doubling the tank size / double the quantity idea, however, will work, as this is down to simple linear proportions. I have serious doubts, too, about the use of baking powder, as this is a mixture of different ingredients, some of which are not really beneficial. I will investigate this and get back to you later on this one. Steve