Ok, so one thing I have noticed. If I have any number of fish in a tank that do not seem to be selling (doesn't seem to matter much if it's 2 fish, 3 fish, or even 5, 6 or more), I exit the Sell Fish mode, wait a second or two, then go back in. Within a short amount of time, I can usually sell 1 or 2 of the fish. I can repeat this usually until I am down to my last fish.

That leads me to believe that the 'customers' have certain purchases 'on their mind' and when you go into the sell screen, you get random customers with random desires. But I haven't confirmed that absolutely yet. Just seems that way. Will have to experiment with it further to see.

I have also noticed that the more fish you have in the tank, the more customers appear in your shop when you go into Sell mode.

I suspect adding tank ornaments has something to do with the minimum number that can appear in your shop, and Advertising research has to do with the Maximum number. Will hopefully be able to confirm that soon.