Throughout the game, you are likely to find various PokéBalls floating around the aquarium. This is treasure for you to collect and there are two different kinds of treasure.

Gold Treasure

The most common treasure that will be seen is the one in the Golden Poké Balls. For every 5 levels your Magikarp grows, a Gold Poké Ball will arrive in the tank. This Gold Poké Ball will only give one gift, coins. The amount of coins you get is dependant upon your Magikarp's level, so Level 5 will only yield a small amount of coins while Level 55 will provide a much larger amount.

This amount can be increased by having the Lampent Lamp decoration.

LevelBase Coin Reward
Level 56
Level 1012
Level 1527
Level 2042
Level 2549
Level 3099
Level 35114
Level 40251
Level 45479
Level 50718
Level 551,351
Level 602,485
Level 653,982
Level 707,456
Level 7513,811
Level 8021,031
Level 8529,678
Sunken Treasure

The other kind of treasure comes in the Red Poké Ball. This treasure is rare and only occasionally appears in the tank, though you can trigger it every 7 hours with Popplio's Support Skill. This treasure gives one of a few random possibilities.

Small Amount of Coins48%
5 Diamonds21%
Large amount of Coins10%
Training Soda3%