I use a liquid hydroponic
nutrient solution called Formula One. I have been using it for years because
I grow plants in all stages of growth from the same nutrient tank. You mix solution A to your tank
and then solution B through the vegetative stage and the bloom stage. Most hydroponic solution has
a two part mix Grow for veg stage and Bloom for fruit producing stage.
The only place I know of to get it is Worm's Way
Worm's Way - Gardening Products, Supplies, Service, Support, and Knowledge Base for all your hydroponics, indoor, outdoor and organic gardening needs, year 'round! They have a store in
Indiana, Florida, and Missouri.
You can use any nutrient solution that says on the label that it's formulated for hydroponics.
The recommend time to change your solution is 5 or 6 weeks. I admit I don't change mine that often
but I like to see how much I can torture my plants just to see how good my system is at keeping
plants growing.
Ron