so for a 12" fish you would have 12' or 144" of space for that one fish? so that would be like 6x2 (180galish) for 1 12" tropical fish?snakehead121;1098584; said:cool guppy, I have used a diffrent method befor. First you find the serface area (length by width). Then for your fish for tropical freshwater 12in. per 1in. of fish, for cold freshwater 28in.(I think it may be 20), and for saltwater 48in. And for fish that go to the serface to breath double the in. requierd.