Changing one thing at a time is a good, time tested approach that can be misleading with planted tanks, as cause and effect are not always intuitive.
As an example, adding fertilizers will not necessary lead to 'better' plant growth if the tank already has enough nutrients for for a given set of conditions. That set of conditions includes light intensity, availability of other nutrients, flow, temperature, and so on.
What helps to untangle this mess is the premise that plants are inhibited / limited by a *single* factor at a time. The challange is to find what that limiting factor is right now.
Understanding the chain of events helps to narrow the field down:
Light -> co2 -> Macros -> Micros -> Flow
Not enough light = plants will not have enough energy to use fertilizers, regardless of how much you dump in.
Enough light - short on co2 = same effect as plants need enough carbon in specific forms before they will need / can make use of / uptake other nutrients.
Enough light + enough co2 = could still = no growth if there is not enough of a *single* macro (either N or K or P). Not enough K = cannot use the extra P or K. (The approximate ratio of *total* available macros is 4:1:1).
And so the dependency / limitations chain continues, one limiting factor at the time.
Hope this helps a bit.
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