I would definitely ditch the fish tank water, aero can be very temperamental. If your going to grow in water then there's some things that HAVE to be considered.
#1- PH, for hydro a safe range is 5.2-6.1. If your not in this range your plants most likely aren't happy. Look up pH lockouts, looks like a deficiency but more nutes never fixes pH problems.
#2- PPM. Ideal range can vary a lot from strain to strain, but with seedlings I would stay less then 300 for sure. growing in water especially you need to have some kind of meter to see what's going on with your plants. If ppm is dropping, your plants are eating. If it rises, you've got your solution too concentrated.
#3- water. General rule for growing in water is get a reverse osmosis system. Then you start with 0 ppm and you know what you are feeding your plants. Stick with a base nutrient made for your growing style.
Not that fish tank water couldn't work, but definitely not a place for someone just getting started IMO.