I would suggest pricing fish at the recommended (starting) price until you have researched Advertising to level 2. From there, you can up the price a bit... maybe raise the price around 25% higher. Once you have Advertising maxed, you can easily price fish for higher amounts.
Also, if you buy the ornaments, you can price even higher yet. Orange Fruitfish is originally listed at $23 (I think), but on the online version, with all the upgrades you can buy, you can sell them for $50 each VERY quickly (I usually can sell 6 at that price before all the customers are even in the store). On the full version, for some reason I do not have that kind of luck. I think I have mine set at $30 or $35, and don't sell as well there as the online version does at $50. I have purchased everything in the game on the Full version as well. It is kind of odd, but that is how it works, it seems.
A good rule of thumb for the selling of fish would be how quickly they sell at the price you have them listed at. If they fly away (like my online $50 orange fruitfish), chances are you can raise the price a bit safely. If you seem to be waiting forever for even one to sell, you may want to drop the price slightly. Pricing seems to be pretty much on an individual basis, and each person seems to get different results.
I personally tend to be impatient, so price mine a bit low, so I don't have to wait through an extended buying sequence