there are alot of factors and its very involved and takes years of research or alternatively you pay someone to do it for you... In aquaculture, densities of .5-.75lbs of fish per gallon are common and high end farms are approaching 1lb of fish per gallon of water with high turnover. Alot of common aquaculture figures are 1-5gallons of water per pound of fish. I could write you a story on here about all the variables.... staying with a common marine ornamental density parameter of .5"-1" of fish per gallon is a safer bet but you will still need to have high water quality to achive this.