Well.......the stuff for aquariums isn't what you want for your plants.....fish products are too high in salts for plants....even freshwater as they simply use different chemicals for the fish adjusters than the plant ones. However it will most likely tell you if you can use it or not by just letting it sit and checking your ph but I wouldn't run it through your system. (ph TESTERS for fish....ok....) Wait a few hours or even overnight and see if the ph stays stable or starts to rise (or lower....). If it stays the same (or a reasonable facsimile (sp?) of) you can use it but if you find that you are constantly having to readjust your water to keep it in a usable range then you should think about r/o or distilled water.

Are you using hydroton or rockwool or ?