I've seen charcoal filters at home depot. I've also read about people making their own. Charcoal filters seem like a pretty easy thing to get your hands on.

Ppm? Don't you wanna get a pH pen?

I think it's also important to realize what connieseur said about micro-nutrients and stuff. You can't just solely use guano tea from propagation to harvest (although that would probably make some damn delicious tasting bud) you might have problems with just guano.

Another really good resource from the organic grow master (soma)
http://www.somaseeds.nl/magazine_art...withGuano.html

If you wanna stay hydro-organic Fox Farm makes a really great Hydroponic 100% organic vegetative fertilzer (Grow Big)