I'd say
K deficiency. I don't see signs of yellowing so I doubt it's iron.
If you don't have to dose much N (from KNO3) then it's very easy to develop a
K deficiency, since we don't have hobby grade test kits for it, we just have to 'dose enough
K' and assume we have good levels until we see problems. (same goes for Iron dosing,
btw).
There's not a lot of
K in KH2PO4 either, when you consider we only need 1-2ppm of
PO4. In fact none of the fert calculators I've seen even will calculate the amount of
K in a solution of KH2PO4.
But I digress...
Tell us all about your dosing routine, tank size, lighting, and what the
CO2 levels are at if you're injecting.