I have never seen this happen in Fish Tycoon. Are you sure they are adult fish? Check to see that the fish are at least age 20. Fish have 3 different stages of growth: the babies (less than age 6) are light gray; fish in the next stage (ages 6-19) have the color and basic shape of adults, but are still pretty small; adult fish are considerably larger than the other 2 stages and have developed fins. If your fish are at least age 20 and your store screen still says there are 0 fish for sale, please report this as a bug in the Technical Issues forum.