I used to empty the buckets and wash them out, then refill with fresh nutrients. But now I just top it off by pouring in another bucket of water with fresh nutrients. I don't actually pour it, I use a water pump to pump the water from the freshly filled bucket into the tubs. Either way works. If you have any water problems, like if it stinks, it would be a good idea to dump the water, wash out the res, and refill with fresh water and nutrients.