It really is up to you if you want to hold onto a Magic Fish or sell it. Some Magic Fish have very positive effects that players like to have in their tanks for a long time, but they also tend to sell at a higher rate of cash than common regular fish, so it really comes down to what you need for your tank, right now.
We have many resources here on the forums for starting players. There is the
Fish Tycoon Guide Forum as well as the Fish Tycoon Forum itself (what we are posting in) that has many questions that were asked and answered over time.
I'm sure you've seen it, but the Search button in the upper right can help narrow down whatever you might be looking for. Good luck & have fun playing!