Well, the water level is an issue isn't it? That's what happens when you use the larger container. How often you change your water depends more on it's condition. Some hydro ferts are so stable that you may not need to change them much. Alot of people like to change once a week, but if ppm and pH remain stable, you might not need to worry about changing them so much. Especially if you're using large containers. Getting rid of alot of water isn't going to be easy. That's one reason why 5 gallon buckets are good to use. Use of more water creates a need to use more ferts which costs you more in the long run. This should work but you'll also need alot of air to keep the roots from drowning. It's really a trial and error kind of process. You've got to do it for a while and when you have something that works, stick with it.