holes in Hygrophila leaves

[juggler]

I have noticed the 4 Hygrophila species in my tank seem to be prone to what look like K deficiency. The bottom leaves are usually pin-holed and more often shedded leaving a bare stem. The plants include H.polysperma, H.corymbosa "angustifolia", two other H.corymbosa sp. The similar looking plant Ludwigia repens don't seem to be affected. Bottom leaves quite intact.
Does anyone else have this problem? Similar pin-holing can be observed in Elatine triandra and E.quadricostatus too. Majority of other plants seem OK.

Is it because these plants are fast growers and they really consumed all the K I dosed?

BTW - my tank specs

++++++++++++++++++
Tank Dimensions (LxWxH): 5'x2'x20"
Lighting Intensity(No of Watts) : 216W (~1.7W/g)
Type of Lighting (FL/PL/MH) : PL
CO2 Injection Rate (bps) : 1/3 (24 hours)
Type of CO2 (DIY/Liquid/Tank) : tank
Substrate Used : JBL+Cocopeat
Liquid Fertilizers Used : Dr Mallick's Lushgro micros, K2SO4
Frequency of fertilization : 20 drops Micros daily. 20ppm K weekly.
Tank Temperature : ~28C


Chemical Properties (Fill what you can)
---------------------------------------
Carbonate Hardness (kh): 2
Total Hardness (gH): n/a
PH : 6.8
NH4 (ppm): n/a
NO2 (ppm): n/a
NO3 (ppm): 20~40
PO4 (ppm): 0.5-1.0
Fe (ppm): 0.1

Bioload (Your Fish and Plants)
------------------------------
Heavy planting. More than average bioload (4 discus, 50 rummy nose, 15 serpae tetra etc).

[wks]

KF, I also suffered from the same problem as yours. Now my Hygrophila are improving as I put in more K and increase kH to 4.