Does anyone know whether customers are more likely to buy fish when you have an ornament in the selling tank? Like for instance, if I were to put the same exact fish in there for sale for the same price on two different games - one with & one without a seashell, would there be any difference?

Ornaments and Advertising research will improve both the speed at which your fish sell and the price you'll get for them. The ornaments increase in effect as you buy the more expensive ones. The shell doesn't do much; the ship has a much more noticeable effect.

I had a shell, a tower and one level of advertising research and the price of a fish went from $15 to $55...