one a week is fine for veg when they wont be drinking as much. eventually they'll probably need more frequent changes as they drink more. one thing to remember, is our ppm meter will tell us how much "stuff" is in the water, but doesn't identify what is in the water. so if you let your soup go too long, your plants may drink use all the nitrogen and hardly any of the other, but your ppm could still be within range cause theres enough other stuff in the water. good luck man, peace
thanks Dorklover and sorry for making you angry with the repeat posts it was a mistake thats an excellent setup you got going on there also