DIY ferts sounds really hard but turned out to be far less complex than I had assumed. One starter question is how things works on the premixed. Each tank has different plants, water, sub and all those other points that change how much we need to add for the right mix of ferts. So a premix sounds easy but it may also miss the mark for different tanks across all the different situations.
So what is a pre mix? It's NPK and some other small bits and pieces that may or may not be needed in your water.
So somebody came up with a different way to go, called EI (Estimative Index) Which is an ESTIMATE of how much each tank might use if fuly dosed plus some extra to avoid running out of anything. Then to keep from letting the tank go way, way high, the idea is to do large weekly water changes to knock the levels back down. Overdosing is not a problem if we keep it somewhat close. I tend to be a bit loose and not too motivated to work it too hard, so I do a modified EI, kind of monitor the levels until a pattern shows of what each tank likes, but all the time it is an estimate, so getting too fussy about exact measurements is not on my path. I see no reason to get EXACT on an ESTIMATE!
I use this to get a starting point but do a bit less than full EI as I don't want to dos eos much that I have to work extra changing water!
I dose NPK on one day by stirring a bit of each in tank water and pouring it in, then on alternate days, I do the minor elements the same and then rest on the seventh day! Or else I get busy and miss a day here and there! Tiny cooking spoons are close enough for larger tanks or just a pinch is good enough for my purpose.
My guide for starting:
Over time, I often see things like you have and want to bump one or another fert up just a bit to see what it does. With dry ferts and being cheap, it is pretty simple to just bump up the N, P, or whatever when something doesn't seem right.
My fert source:
Looking at how much cheaper it is than premixed, I don't mind just a bit more trial and error than some like. In my case, I have extreme calcium and tried adding mg but found adding iron was the more helpful thing.