I think that the frequency of purchasing is somewhat random. yes it has to do with the price of the fish and the amount of fish in the tank, but I have had this happen with fish prices set at their most expensive level.

Since the only thing that that controls is frequency, Then the fish could be sold at any point in that time range. for example if the prices and number of fish I have in my tank would lead to a customer about every 10 seconds, that customer could buy at any point in those first 10 seconds...second 0.1 or second 9.9 . Once that fish is sold, I think frequency is recalculated and you can get another fish sold an appropriate time later, but from my experience that initial sale can come at any point.

My best guess...