There are 20 fish in a fish tank, including goldfish 15, red fish 6 and snakehead.

Red fish is a kind of goldfish, while snakehead and goldfish are two kinds, so excluding goldfish is the number of snakehead, so the number of snakehead is 20- 15=5.

According to geomantic omen, it is better to keep a snakehead in a fish tank, in which 1 is white, 2 is black, 3 is green, 4 is blue, 5 is yellow, 6 is white, 7 is red, 8 is white and 9 is purple. Black is unlucky, and the number is cloudy. Generally, a fish tank and a fish are enough. In addition, there should not be too many red fish. The water in a fish tank is something that can create wealth. Red is a conflicted fire, while white and color are better. In addition, there are eight fish characters.