Art_Giacosa said:
I am a strong believer in newcomers using certain test kits to understand their aquariums.
I am a strong believer in the opposite. :

uts on boxing gloves::

Aside from the KH and pH test kits, the rest are very unnecessary.
What newbies need is a lesson in plant nutrition and how it relates to the water chemistry of the planted tank. Test kits do not teach them this. All it does is promote a dependence on numbers that often make no sense to them. Furthermore, good test kits are expensive and the color charts are enough to drive one bonkers. The money is better spent on other equipments such as light and CO2 system which many of them usually neglect until they have exhausted all other options. <-- biggest newbie mistake IMO.
Even if they have taken care of lighting and CO2, test kits still aren't necessary. A free comprehensive water report from the local water plant is more important than those expensive test kits. Coupled with Chuck's calculator, they can dose NPK/GH/micros into the recommended ranges and get excellent plant growth.
IMHO, test kits first become important when the tank experiences problems that have been determined not to be caused by light and/or CO2. The succession goes: Light --> CO2 --> N --> GH --> K --> micronutrients --> P.
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Keeping ratios of 10:1:10 constantly is extremely impractical IMO. The monetary costs and extra work involved in maintaining such a ratio have not been justified. As long as adequate nutrients are available, does it matter what ratios they are in at any given moment, given the wide range of tolerable concentrations?