This looks like a macronutrient issue to me, Phosphate probably.
Definitely could be, but it could be any of the other nutrients. This is an <"analysis of ADA liquid fertilisers
start with aquavitro so hopefully this is now covered
Is it ADA substrate? Even if it is I'd try something like <"TNC Complete
"> or <"APFUK Complete
">. They are a cheaper option, I'm not familiar with Aquavitro fertilizers.
If you haven't got any fish? You can use a complete liquid feed for terrestrial plants, it doesn't matter if the nitrogen source is ammonia or urea.
Plants can only make use of the extra CO2, if everything else is available. Plants can only take up nutrients as ions, and every K+ or NO3- ion is the same as every other K+ or NO3- ion.
All the elements on the Earth heavier than helium (He) have been here for the last 4.5 billion years, so the reactive elements like oxygen and potassium have been combined in hundreds and hundreds of different compounds.