First, you need to know how big the aquarium you bought is and whether it is suitable for raising koi. Koi belongs to large fish. The body length of 2-3-year-old fish can reach 30-40 centimeters. If the fish tank you bought is too small, it will be difficult to raise good koi. The main characteristics and concepts of koi have been said by people upstairs. I started to learn how to raise Japanese (red and white) koi with an aquarium ten years ago.