One thing that makes fish sell much faster is to make sure that your selling tank is very full of fishies before you click the Sell button. The more fish you have, the more customers there will be, and the faster you will sell. You don't say what level of Fish Environment you have, but there are several fish that will remain healthy in even a level one environment and bring prices as high as $65 or more. One of those is the first Magic Fish, which you can get from common eggs. The suggested price that the game provides for a fish is usually a good starting point, although you can usually sell the fish for more. It's a trial-and-error process to determine what the maximum asking price for a particular species actually is.

If you'd like some spoiler kind of help, just send me a PM and I'd be happy to provide you with specific "recipes" for some good fishies!