Sorry, I just found this article, and I'm mixing up kH and gH:
http://www.drhelm.com/aquarium/chemistry.html
My pH is not swinging daily, despite the kH being so low, but Hamilton water is soft and acidic, so having low kH means my pH drops over time with several water changes. I've been keeping records and have observed this.
I've been looking at filshprofiles.com, and each fish has a recommended gH range (expressed in units of dH just to confuse noobs like me), e.g
http://www.fishprofiles.com/files/profiles/321.htm
This fits in with my observations - I've noticed my tiger barbs and siamese algae eaters start flashing a bit when the gH is below 4 dH. I add Sera mineral salt to get it back up to around 7 dH and the fish stop flashing. So I'm looking for a way to keep my gH at the recommended levels without having to buy the Sera product, which I'm sure is hugely marked up. If I could find the ingredients and ratios, I could make a poor man's mineral salt
simian & tHE cONCH, since the plants are consuming the minerals, surely they need them and have an optimal level as well?
Suphew, I have a couple of large bits of wood in the tank. How do they soften water, do they absorb the minerals? If so, hopefully they'll get saturated eventually?
purplecatfish, sorry but I think you're confusing ferts with minerals, I'm only asking about kH, pH and dH, oops, I mean gH Or does PMDD control gH as well?