In a perfect world...you would clean the water/gunk out of the bottom of each bucket after draining out the old water. Being in a recirculating system this may not be so feasable to do though. If you cannot get down their and get that last bit of dirty water out, you could always fill it up with clean water, drain it again, and if you're obsessive like me maybe repeat that process a couple times to rinse old nutes off the roots and sort of rinse the salt build up on the bottom out. With that being said....if you're adding back nutrients along with the water you top back off with to keep the PPM's where they need to be, you can go longer than seven days between res changes...personally...I go between 15-30 days between res changes. But when I add water back, I add all of my additives and base nutes back at whatever strength I feel the plants can handle...figure out what works for you...res changes can cause more stress on a plant than you think. If your PH is stable, then I wouldn't worry about changing the water...if their is an issue with your res water, the PH will indicate it...if the res water is stable, your PH will be as well...Keep in mind, I use beneficial bacteria, or else I would need to do res changes more often...good luck bud hope all works out!