Wait until your fish mature before selling them. Althought selecting the rapidly maturing Mini Dart Goby fish is an early and cheap option for only 9 Coins, choose fish that take longer to mature. For example, the Scooter Blenny can be bought for 88 Coins and grows every 24 hours. When sold it is worth 221 Coins. Along with coins, the Blenny also brings in a lot experience.