In Koichan.io, you have to help your fish stay alive and grow big and pretty.
The point of Koichan.io is to get bigger fish, level up, and get new skins. Get color dots, which are plankton, to get points and get bigger.
Take note that some of the dots are dangerous and should not be eaten. You can't eat green plankton. In the bottom left area, there may be a health sign. Once you eat a deadly dot, your health will start to get worse. When you have no life left, you die. To keep your health from getting worse, you need to find and eat a cure (white dots).
Your health will slowly get better after you stop poisons. Some of the dots probably give you brief skills like boosting.
Have fun and good luck!