Hi, I saw on your site that you said 'Salt can be added as a general tonic at about one Tablespoon per five gallons.', I was wondering if this can be done to ponds with plants, specifically water hyacinths and/or watercress?

Thanks :-)

That amount is what I use for aquariums including ones with plants. In ponds, I use much less. I think it is 0.1 % salt that they recommend for fish only ponds but I probably use less than a tenth of that. Plants don't like more than about 0.05%. I'm bad and don't really measure the salt anymore, just broadcasting small amounts in the spring into my pond, never getting very much in there.
See http://www.fishpondinfo.com/chem.htm#conver under examples for how to calculate % salt needed for a pond.