Hi,
The more plants get fed the more they uptake and the more waste they produce. This then requires more water changes to keep the tank clean and to keep both plants and fish healthy. In this sense fine tuning is better associated with lowering the maintenance requirements. Lower dosing also means lower cost, so fine tuning has an economic component. I agree therefore (and often suggest) that a reduction from the baseline dosing numbers while watching for deficiencies is a good thing to do if maintenance requirements and economics are important. HOWEVER, I absolutely and positively have never suggested that anyone use delusional test kits to determine this reduction from baseline. Of all test kits on this planet, NO3 test kits are THE most delusional and despicable.
The proper technique for dosage reduction is to drop some arbitrary percentage level, such as 10% and wait 3 weeks, then to continue this pattern until you start to see the beginnings of deficiency. This automatically takes into account the Dissolved Organic Nitrogen (DON), so there is no reason whatsoever to waste more time and money looking at a lie. Your tank is your NO3 test kit. When you start to see yellowing of leaves or BGA then you know that you don't have enough NO3 and that you should revert to at least the previous dosing level. You do not need to know what ppm this is because the ppm of NO3 that causes deficiency varies considerably from one tank to the next, and even varies considerably in the same tank based on plant biomass, flow/distribution, lighting fish stocking level, maintenance practices and CO2 injection technique. Therefore, knowing a ppm number for this tank this week is completely meaningless because it tells you nothing about the past or the future and cannot be used as a frame of reference for any other tank, or for the same tank at any other time. The measured value is completely meaningless and irrelevant. Once you understand this at a fundamental level you will realize that knowing ppm numbers does not help you to grow better plants.
You also need to be systematic in your approach to interpreting your observations. There are many factors and variables in the tank. You cannot conclude that an arbitrary KH value necessarily grows crypts better than some other arbitrary number without further corroboration, even though it might actually be true, it is not a logical conclusion because we do not know what other variable changes occurred with your change to higher KH water. To verify your conclusion you now must use, for example low KH water for your water changes while keeping all other parameters the same (as much as possible) and see if the crypts revert to their previous behaviour of paler colours and less vigorous growth. If you draw a conclusion about some environmental factor's effect on plant growth then you need to be able to show specific cause and effect and it must be demonstrable as well as repeatable. What you observed could all too easily be a coincidence. There could have been something else that you did to the tank to generate the change in physiology which accompanied the colour and growth changes. Very few people perform this rigorous procedure. They make a variety of changes to the tank and arbitrarily attribute the effects to only one of the variables. Again, I'm not doubting your conclusion - what you observe may very well be true, but you have to be able to prove it because otherwise you will only be generating another myth, and there are far too many myths around as it is.
Cheers,
thanks for that I was always under the impression that as long as there was enough ferts the plants would just use what they needed, I never realised that having higher levels encourages them to take up more so that's another pearl 
