I used General Hydroponics when I first started with a small system.
( see link) It worked well for me but it can be pricey for larger systems. Now I use 25 # bags of powdered fertilizer meant for hydroponics because I have a larger system. The bags will last me several years but are more cost effective.

Here is a link that might be useful: Hydro Fertilizer

The General Organics Starter Kit is more for use with soil. I would suggest the General Hydroponics Flora Series Performance Pack (link below). It comes with everything you need to get started and is perfect for hydroponics!

Here is a link that might be useful: Performance Pack