Be sure to waituntil your fish mature before selling them. Although the rapidly maturing Mini Dart Goby fish is a quick and cheap option at only 9 Coins, instead pick fish that take longer to mature. For example, the Scooter Blenny can be bought for 88 Coins and grows every 24 hours. When sold mature, it is worth 221 Coins. Along with coins, the Blenny also gives lots of experience.
-Buying fish eggs also gets you experience. For example, buying a Shy Hamlet for 35 Coins gains you 15 experience points. A Scooter Blenny gives you 29 experience points. If you want more experience points and have enough money, buy the more expensive fish.
-When waiting for your fish to mature enough to sell, visit a friend to help clean their tank or adjust their volume so you get more Coins and experience points. Make plenty of friends since you can only help each friend once per day.
-Sardines take only three minutes to grow to junior and around ten minutes or more to become an adult. They require more feeding but result in fast experience. When they reach adulthood, sell them for 12 experience points and 48 Coins.