I am not advocating any more than two tenths variances in any 24 hour period, as detailed in another post. To get from 6.0 to 7.6 would require; calculation of the entire dose required of each compound for the entire net volume of the tank, then dividing that dose by eight, this would ensure that no more than 0.2 increases per day to avoid ph shock potentiality. When the entire tank is at the desired value i.e. 7.6, then the full dosing rate would be used of each compound for the given water volume being replaced per water change after that point. In any event I would never endorse 50% water changes unless there was an acute problem/emergency (poisoning etc). I have had many XXL fish over a 30 year period (31.5" RTC, Arowanas, xl Cichlids, Snakeheads) and have never gone over 25% at a single change. As a result I have always had nitrate readings between zero and 20ppm (max). If I wanted to change more per week I would increase the FREQUENCY to 2 or maybe 3 times per week, not the volume.
Then again I use several filtration types, typically external/s plus sump, very high outputs/turnovers, excellent media, plus ozone controlled via a mv/redox computer, over and above high volume and very frequent water changes. The pH is monitored digitally daily and cross referenced against the most accurate test kits in the world, AquaMerck. The trick for early determination of a forthcoming water change for me is primarily pH; when it begins to drop, even very, very slightly over 2 consecutive days then that's an indication of denitrification, on a secondary level a change of the mv/redox reading on the computer also indicates increased denitrification. The need for me to test for nitrate is therefore negligibile, it's only done once per week (AquaMerck) as a cross check and put in my diary/log. You are correct that most people do not have this problem, usually this problem only arises when either people are keeping Rift Lake Cichlids or (like the questioner) are unfortunate to have a wide variance between their tapwater and the desired value they wish to achieve. Hope this makes sense.
