Oh. . . like a mini fuge or something right? If I add a fuge (that I dont need) to my nano, it will be atleast 10 gallons. I also fail to see how bioballs are more effective than LIVE ROCK rubble. You can fill the bio cube to the top with rubble which allows you a little bit more water volume. If you go pass the MAX line in the biocube, you submerge the bio balls & the bacteria on them need exposure to air. That is why it's called a trickle filter. If you fully submerge bioballs, you cut there effectiveness down to 30% at best.