You need to use 30 gal without the insert and 25 with the insert you could still use 30 gallons if you want though. As for how often you do a res change it will depend on what stage of growth your plants are in and how many are in the tray. A ppm meteor comes in handy for determining when to change the res. I have 15 in my tray I had them at about 800 ppm and it would go down about 30-50 a day. Also Another way to know when you need to change the res is by keeping track of how much water is used for toping off the res so if you use 30 gal res you should only use 15 gallons of top off water before its time to Chang. I just use the ppm meteor and keep my solution fresh so the nuts do not get toxic levels.