Grammar problem
I have heard plural form of 'fish' is also 'fish' when talking about a single species of fish and plural becomes 'fishes' when talking about different species of fish. But when we have number of fish of one species for ex. 5 and also some more fish of another species for ex.7 , how do we choose the plural to describe the amount of fish in the tank?
There are 12 fish/fishes in the tank. -: fish or fishes?

And I have heard plural of 'goldfish' can be either 'goldfish' or 'goldfishes'. Are those both ways valid to make the plural of any species of fish using its name?

Someone please help me.

Question

Grammar problem
I have heard plural form of 'fish' is also 'fish' when talking about a single species of fish and plural becomes 'fishes' when talking about different species of fish. But when we have number of fish of one species for ex. 5 and also some more fish of another species for ex.7 , how do we choose the plural to describe the amount of fish in the tank?
There are 12 fish/fishes in the tank. -: fish or fishes?

And I have heard plural of 'goldfish' can be either 'goldfish' or 'goldfishes'. Are those both ways valid to make the plural of any species of fish using its name?

Someone please help me.

Answer

This can be a confusing problem, both for native speakers and learners of English, as both plurals can be used.  Most commonly, the plural of fish is fish, whether we are talking about individuals or species.  So we have 12 fish (individual) in the tank.  We have two species of fish in the tank.  We don't have two fishes in the tank. Although we can use the term fishes to refer to species, it would be used more in the sense of 'all the fishes of the world' or 'the native fishes of x-land'.  Otherwise it could be confusing, and sounds wrong to many people. Sorry, I can't give a more definitive answer.

Answer

Use fish when talking about a number of fish, regardless of species. Use fishes when only talking about species.
I caught five fish.
There are four fish in the tank.
The fishes in the river are salmon and sturgeon.

Just use goldfish -- it's probably the simplest way to go.