There seem to be a lot of commercial all in one plant fertilisers out there (and several DIY recipes) so why do some folks use separate Macro and Micro solutions?
Because they can not always be mixed into 1 solution at least not in liquid form.. And if you do mix them in at the same time some elements will instandly bind to eachother into inert substances unavailable to plants. For example Iron seems to bind to Phosphor and renders it useless into a unavailable different molecul for plants.
Most all in one fertilizers are not realy all in one all the way. These fertilizers are usualy meant for organic soil types that already contain (store) micros in organic ways. Such as composted organic carbon based soils (substrates).. these kinds of substrates have a buffering capacity, hold, (bio-)convert and release trace elements due time. Fertilizer for these kind of soils contain NPK, Mg and maybe some Ca. For most generaly used soils this is considerd a complete mix.
But if you grow plants on an (relative) inert inorganic substrate than you need the whole package.. And this can never be mixed as complete for the above given reason. This always comes in separate containers named "Macro" containing
N(itrogen)
P(hosphor)
K(alium) (K is in english Potassium) as main building block and "Micro" with Fe, Sn, Cu etc. etc. as sub main building block. It also is best to never add both at the same time for the above reasons. You should wait at least 24 hours adding micros after the macros. Again for the above given reason.
Distracting is naturaly backed clay soils, are in origine organic, but backed for example by vulcanic activity and is rendered inert by this process. Thus inert soil from organic origine. In most cases this is what is used in the aqaurium hobby and if not enriched with fertilizers by the manufacturer it simply is pretty inert. Than if you want to add the whole package you need a Macro and a Micro fert source.
Tho an aquarium is a biological organic entity, plants grow and die, fish eat and poop etc. Also this provdes a number of elements that can be converted back to plant food. This will build up initialy as detritus in the soil and than is bio converted back to usefull fertilization elements. Also this is taken into consideration for a manufacturer to call a fert solution complete. Under assumption the aqauarium provides by natural means what the package doesn't contain.
Nobody said it was easy.. Not easy to get your head around nor easy to explain in layman's terms.. But in a nut shell this is about it. Tho rather thick books with a number of mind bogling formulas are writen about it.