First grow question, how often to add/change nutes in reservoir

I've got ten plants in smartpots w/coco, on a drain table. I'm drip feeding the plants for fifteen minutes three times a day out of a five-gallon reservoir. The plants are about 3" tall now and appear to my novice eye to be doing mostly okay. I'm using Canna brand nutrients with the schedule tool at http://www.cannagardening.com/growguide

When I started the nutrients a week ago, I added Canna A, B, and Rhizo to five gallons of distilled water in the reservoir. Now, there's about 3.5 gallons of fluid left in the reservoir. I know the plants have taken up some of this, but I assume I'm also losing some water to evaporation.

My question is, what do I add to the reservoir to keep it at five gallons? Do I add just distilled water, or do I also add some more nutrients? Also, when the schedule says to change the mix of nutrients, do I have to empty the reservoir and start fresh, or can I just add the new nutrients on top of what's already in the reservoir?

Thanks!

Your going to need a larger res. I would go to an 80 litre / 21 gallon @ minimum. Your plants are going to start drinking much more than that soon. At that time you will have to fill every day or two. Plus you dont want to run your pump dry.

once you have a more appropriate rez, one you dont have to fill all week, I would be fully changing (draining and rinsing then adding fresh nutrients) your solution at least once every two weeks, topping up with fresh solution weekly keeping the ppm in the range you want according to what period of the grow your in. Some guys do a full change every week. Im sure someone will chime in soon and tell you all about how you have to change out once a week.

want more answers/clarity? "reply" to the post or I wont see it. others will be around as well.

Your going to want a PPM meter and PH testing strips or device. Otherwise its like poking your dick in a hole. Maybe theres a woman on the other side. Maybe a man. Maybe a horse. Maybe nothing. You'll never know what the fucks going on.

Thanks
@Morbid Angel that answers a lot of my questions. I've got a pH test kit and I've been keeping it around 6.0, but I do not have a PPM meter. Is there a particular meter you would recommend?

Well.... I use Bluelab. But there are others. Check out what your local hydro store has.