Well, I will try to answer both in one response...
for hydroponics it is imperative you KNOW what your hydroponic solution consists of. Number 1 water
Number 2 nutrients

In order to know what your water is like, and know how much nutrients you have in your hydroponic systems, you need measuring devices, or meters. If you want to be successful in hydroponics you have to have them. I don't care what brand or make, but there is no other way but to purchase a meter for your water, and your nutrients. I would say you can get both for about $25 each. The one that measures the acidity of your water is the PH meter. Sterilized water has a PH of 7, which is known as 'neutral'. For our plants we need a lower PH, especially in seedlings and very young plants. The meter that reads the nutrients is called EC/PPM meter. This meter can't tell you have "Big bud" nutrients in there or any other fancy name that the grow shop says you need. All it tells you that distilled water has a PPM (parts per million) of 0. PPMs are needed in each phase of the plant as it goes thru the seedling stage thru to harvest time.

Seedlings: From the very day you set a seed out, you have to keep it most, warm and in water with a PH of around 5.6 give or take .5
There should be no nutrients added to the water until they have developed roots. That means your PPM meter should not be any higher than say 100

Young plants in the 18/6 stage, that is veg room:
Your water should be in the high 5 PH range... something like 5.7 would be ideal. The PPM should be around the 600 range.

Flowering plants would have an ideal PH of about 6
PH
The PPMs of flowering changes thru its say, 2 months it is in flowering, and everybody has different theories. I will try to give a median, for anyone to start on, and then you could change it according to your taste, or strain or whatever you want. I will go thru my stages, that I use. I use only GH 3 part, and don't believe in the expensive 'boosters' to bolster your yield, or is it their pocket book?

When my plants come out of the veg room they have been used to 600PPMs. So now they've been transplanted into a larger pot, their light is changed from a metal halide lamp to now going under high pressure sodium light. This is a stressful time for the plant, and I generally start them off around 800 PPM and I make sure the lights aren't bearing down on them in the first 2 or 3 days. Then I lower the lights. After two weeks, I drain the nutrients, and give them all new fresh nutes, this time the PPM goes up to 1000-1100PPM. Two weeks after that I drain the nutes that are left and give fresh nutes 1200-1400PPM. and the last time they get drained of its nutrients, and are running on just plain water, no nutes at all. This is flushing, and takes out the chemical taste of the nutrients. You will probably never get the PPM to 0, simply because the roots have been in a high PPM environment.

sn