You should change the water/nute solution every 2 weeks. When it comes to how many plants per what size container I would saw 2 in a 18 gal 1 in a 5 gal. They need to have the space to fill out, or esle they will grow into each other. As far as I know there is no plant to water ratio, there are different opinions from different people. As far as I know when starting at a seedling bring the water up to about the bottom of the net pot, so the mist from the air bubbles will water the base of the plant. Once roots have developed drop the water level to about 4" below the pots. Hope this helps, I myself know little of hydroponics, but this is what I've learned thus far.

Respect,
G_V

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