Well I did a check (never believe what people tell you, unless you checked it yourself!) and it is true.
I kept the Mini Dart Goby (got it from the Market and not the Fish Mastery thingie). It normally sells for $18 Coins at 1year old (see orange pictures, I cropped it from the Market) but today the fish can be sold for $36 coins. I don't know about the XP part but it should be true too (some extra XPs for you).
Read my post on what are the type of fishes you should be getting and my list of updated list of fishes to buy if you need more tips.
You may want to view my other FishVille posts or surf around my blog. If you like, please visit my Support page.
Share and learn. Learn and share :)