Well you won't be filling a tank and leaving them to drink from it the whole time. You have to adjust pH and ppm pretty frequently.

I've grown lettuce and spinach in an 18 gallon tote, it works fine. You could go the whole grow without a reservoir change (I've done it, don't recommend it) but generally you want to dump it all and refill with brand-new solution every couple weeks at least.

Is it a waste? Perhaps, but it's better to waste some solution than to kill your plants. They drink different nutrients at different rates, so after awhile your solution will register as still being plenty strong, but the necessary nutrients are gone and the ones that aren't needed as much are reaching toxic levels.

The big operations run a continuous system with no waste, but they also do in-house tissue sampling and on-the-fly customization of the nutrient solution. They can add specific ingredients to balance things back out, where we can't because we just don't have the resources or lab.

As far as the simple answer goes, it varies a lot. I'd say you will probably see around 5 gallons per plant total loss, but it will vary.

Hydroponica, thanks for your reply!

If I need to dump it every couple of weeks, should I try to use a shallower container instead of a 18-gallon tote?

But if each plant is taking about 5 gallons and I can do about 12 plants on the tote, it doesn't seem too much waste....

(I guess I really have to just try it out to see how it works).