Likely not an issue of deficiency in the ferts, rather, CO2 competition with other plant species, too much light etc.
PO4 should build and N should be the most limiting fert, so yellowing of the faster green plants, floating species etc is far more typical.
With Excel, I would dose KNO3.
2x a week dosing is plenty, maybe once a week etc.
1/4 th EI should be fine.