I keep mine filled, and don't have much of a problem with fish loss. The less water, the less disolved oxygen, and the quicker other harmful compounds like amonia will build up in the water. Also the lower the water is, the less stable it is in terms of temperature. Recirculate will help a little, but it is like breathing in and out of a paper bag - it is the same water being sucked in a spit out. Pumping out and re-filling is the best way to keep water quality at a better level - this brings in new/fresh water. The more fish you have the more often you should do this. And after runs in rough water, some of your water will be lost to the overflow drain and should be filled back up. The more fish you get in there the more often you should tend to the water. I weighed in five good ones today, and I had to put some work into keeping the water and in turn the fish healthy. I probalby went through 5 water changes in the last 2 and 1/2 hours - they all were released alive.