You could always use the old stand by "We can make some money by buying small and selling large" that seems to work roughly 50% of the time. The only draw back is when the fish are fully grown and colored up, you don't really want to get rid of them. So the next old stand by "Hey we can make even more money if we breed fish". Then you can justify adding more tanks for the breeding process. This however tends to lead to a call to a lawyer more times than not...
Good Luck !