Calculate the formula according to the meaning of the question:

Red goldfish +3= black goldfish

Red goldfish =5

Available:

Black goldfish =8

Total: 8+5= 13

So there are two kinds of fish: 13.

The nature of addition calculation:

In addition calculation, two numbers with the same sign are added, the same sign as the addend is taken, and the absolute values are added. When two numbers with different signs are added, take the sign of the addend with the largest absolute value, subtract the number with the smaller absolute value from the number with the larger absolute value, and add 0 to any number to still get the original number.

For example, * * * has a combination of three apples and two apples, and * * * counts as five apples. This observation is equivalent to the mathematical expression "3+2 = 5", that is, "3 plus 2 equals 5".