Everyone is going to have an opinion on this one and several on this
N/G that know far more than I, however I'll share my recipe. First, I
run well water, which may provide an especially high nutrient value,
but I've rarely had a problem with a deficiency. I dose nutrients to my
15 gallon (25%) water change about every 10 days.

5mL Serra plant fertilizer
1.5 tsp Green Light Stump remover (NO3) to achieve 5-10ppm (cause I
have a low fish load)
2 tsp baking soda for the KH to achieve 5-7dKH (cause my water is
really, really hard)
1 tsp Epsom salts and sometimes I add "ph7.0" product for a low level
of phosphates.
and three Serra substrate tablets under my swords once a month.

IMHO..... every case if different so don't follow my recipe exactly.
You will need to test your water often for several months to get a
feeling for the condition of your tap water and your tank's nutrient
use (or accumulation) dependant on fish load, plant density, substrate,
gravel vac method, ect, ect. Start out testing once a week. Once you
balance the environment.... once a month?

My Aquaria info & pics at:
http://www.geocities.com/spiral_72/Spirals_page.html