First of all, Bauks, you are correct in saying that silver is not necessary in growing plants. And likewise silver is not essential for humans to consume in any form. Silver is, however, beneficial to both. It is found quite commonly as a trace element in the soil across the globe and, in turn, trace amounts are absorbed by plants. Modern farming has depleted most of the silver out of farmland due to it's continuous process through the years. By consuming fruits and vegetables we took in silver in days of old. If you read about the nature of colloidal silver and how it works on single cell microbes, you will gain understanding of why I think this reservoir move is effective.
Zem, I encourage you to do your own research on the health benefits of silver. Beware of the purple guy in the video. He admits that he started turning blue when he rubbed "colloidal silver" on his skin. It is more difficult to find videos of him saying that, and even harder to find him saying that it was not colloidal silver, but colloidal silver in the form of a silver protein. A silver protein product can be classified as colloidal silver, but it is much more concentrated and has much larger particles. These particles would normally fall out of a distilled water colloidal solution. To keep them suspended the manufacturers use gelatin. These gelatin solutions can have 1000 ppm of silver or more! I've heard of some containing 30,000 ppm. Get it? The more I read about this guy, the more I think he wanted to turn blue, or was paid a lot of money to turn blue and is now the poster boy for the anti-colloidal silver movement. He also admits that he still drinks colloidal silver for it's health benefits.
And Bauks, The FDA won't touch it because no large financier stands to profit from it. It is too easy and inexpensive to make at home in this modern age and is not worth the trouble or the money for them to go through the official testing/certification process, nor do they want to. Their big pharma buddies would be in for some serious losses. If you do your research, you will find that people consumed silver all the time for medical purposes until 1938 when penicillin and other antibiotics hit the scene. On a side note, it is also recommended to purify water in cisterns. That and it's benefit to plants is what led me down this path.
I will also go out on a limb to say that beneficial bacteria in a hydroponic system is of little consequence. I say this under hypothesis that plants will suck up whatever solution they are given. That stated, breaking down organic compounds into something that the plant can more easily take up is not necessary when the solution provides what they need already. Adding silver to the solution may be purely beneficial in these terms...I have a feeling I'm going to get some responses from this statement. One tid-bit to consider before responding: if plants could pick and choose what they wanted out of a solution, then why would they get nute-burn?