Nutrient Deficiency?

This looks like a nutrient deficiency, but I'm not sure.

My c. wendtii has leaves with holes, and jagged leaf tips and edges.  This is
not the ubiquitous cryptocoryne melt. The leaves don't get mushy, they just
slowly fall apart from the edges.  The hygrophilia spp. (my usual early
warning indicators for K deficiency) are doing great.  Fertilizer regimen is
2 ml PMDD daily (no NO3 unless nitrates test 0 ppm), with pond tablets
(10/15/10 w/ trace elements) pushed under the substrate every six to eight
weeks or whenever anything seems to need a boost from below.

Tank specifications:  20 gallons / laterite substrate / pH 7.0 / 12dGH / 12
dKH / 0-10ppm NO3 / 45 watts full spectrum light (11 hours) / DIY CO2 / 4
platys, 5 rasboras, 2 SAEs, 1 Cory / PMDD (2 ml daily)

If this is a potassium deficiency, how much K can I safely add to a 20 gallon
tank?  My water changes are 20% weekly (tap water cut with 25% RO water since
our water is 12 dGH out of the tap).

Your expert advice would be most appreciated.  These used to be very
beautiful crypts.

Bill Cwirla
Hacienda Heights