CM...I'm sorry if I came off rude.....I'm an old curmedgeon...didn't mean to be a crab.

I think that is definitely your best bet here. That 420ppm in your tap water....just plain scares me. That stuff could be ANYTHING in there.....we don't want it clogging up your grow and for all we know WHATEVER it is it could be locking out some of your needed minerals etc...

Stinky's point about hydroton is also something not to be taken lightly. I had issues with it as well and you have to watch your ph very carefully until it stabilizes. Interestingly though, only some people seem to have this problem with it.....I had horrible problems and gave the stuff to my friend (the rest of the bag..unused) which she turned around and used with absolutely NO ph issues at all.....EVER from the VERY same bag!!!

I'm afraid I'm not familiar with the Earth Juice nutes so cannot really give you a starting point for them. I use Botanicare ProGrow (a one part grow nute) so you may want to ask your hydro guy what is a good number to start with. Some of these 3 parts nutes are measured out differently....which is why i hesitate to tell you to take a 3 part nute to "200ppms". I just don't know enough about them I'm afraid to give you precise directions on their use. Perhaps someone else here uses them......and will know.

PS....Good luck. You're not too far off.....but are making things more complicated than they have to be...hydro is pretty self sufficient if you have all the working parts together correctly. That's one of the reasons I like the one part nutes....anything that's easier and more convenient works for me. You'll get there.....don't worry.