Brown spots on Hygro.

After increasing my light levels, my hygro. poly. has put out copious new 
growth.  The leaves are developing small brown spots which develop into 
holes, however.  This is causing a bell to ring kind of faintly, but I 
can't recall what this indicates.  I am thinking potassium deficiency, 
but can't recall for sure.

I fertilize with tetra flora pride at half the recommended dosage (I 
figured I could increase if needed, like now, maybe) and just (2 weeks 
ago) placed some Duplarit K laterite balls in the substrate.

Any help is appreciated; I feel like I should know this one . . .