My 2 yr old pond still presents a conundrum. 500 gallons, full sun, z5,
readings always within range, ammonia and nitrate always read zero, never
any suspended algae, but SA, clear water, under my fish load, don't feed
them, lots of plants of all types, 75% coverage, K, iron and magnesium (low
in my area) added regularly, the K weekly along w/BZT.

But...

The marginals never grow well. The lilies I fertilize, and the WH seem
fine, but the irises, arrowroot, blue pickerel, equisetum (dwarf horsetail),
all in pots of clay + pebbles, never grow well. Leaves are puny, older
leaves yellow. I remember a thread a year or so ago about nitrate-limited
ponds and adding potassium nitrate, which can't be legally purchased in my
area. If these plants were in the soil, I'd say they're missing nitrogen.

Is there any other N source I could try out on the pond (carefully)? I
thought the growing goldfish population would change that this year, but it
didn't. Any suggestions?