Nitrates are baaaad. Well not for plants
They are one of the macro nutrients, and if there aren't any, plants will grow thin, pale and slow.
In non-planted tanks, NO3 levels can accumulate, and therefore they are considered a bad thing, which is relieved with water changes (or -yuck- chemicals). In a planted tank, especially with lots of light and plants, nitrates tend to run out, which isn't good because it limits plant growth and can lead to algae outbreaks.
I think levels less than 20 ppm NO3 are great for most fish, although some more sensitive ones like discus (corys... could be) like them even lower. The usual recommendation for planted tanks is 5-10 ppm, no harm to any fishies. Keep in mind that for us, the gov has determined that it is safe to drink up to 45 ppm :shock:
Phosphate is another one of those macros, and if the test kit is right and they are zero in your tank, it might be good to carefully bring them up just a notch. Do some training in a bucket before going wild in your tank
Seems like potassium is under control... daily dosing?? I am so lazy... I dose once a week, and often skip a week (for potassium!). I think it gets stored in plant tissue, and once healthy levels are reached, one can step down a notch.