Thanks guys - I should probably look to cut back on the ferts then. AFAIK my tap water has sufficient N (18mg/l from the water report), K and Mg (I'm in an area where Mg is present in the tap water), and I guess a fair bit of N and P will be added via fish food/waste. If I'm changing water every day currently, is it likely the tap water will be providing sufficient micros also?
The plants are mainly anubias, crypts, ferns and some Buce, so all very slow growers other than a bit of eleocharis and lilaeopsis brasiliensis at the front.