Reverse-Osmosis water, (like distilled water), is really not needed, or generally recommended ... it takes out too much ... you have to add calcium, magnesium, iron, potassium, etc... to make up for it ... if you have 40ppm, like the originator of this thread, you're in great shape, to begin with ... the only thing the average grower wants to remove, is chlorine, anyway ... chlorine will evaporate from standing water overnight, and very quickly, if an air bubbler is run thru it ... chlorine doesn't hurt your plants, by the way ... in fact, I use straight chlorinated tapwater when cloning, and with young, beginning seed-sprouts ... the chlorine eliminates some concerns of funguses, etc ...
... chlorine does, however, kill beneficial bacteria in the soil/solution (basically, the bacteria helps break down the nutrients, so the plant can absorb them) ... these can be replenished, with the use of 'live culture' additives ... I use Fox Farms' Big Bloom, there are others, too ... :smokin: