i dont really know what kind of fish i have its black and got really big ! how do i know if it is pregnant?

Answers:
It's probably a Black Molly. They are a common livebearer in the hobby and females will tend to have babies every one to two months. Females will have a fan-shaped anal fin, while the anal fin of a male is folded into a gonopodium (they use this for mating). Males and females are usually kept together in the stores, so if your fish is a female it is very likely pregnant. It is best not to try and save the babies from the parents and other fish. They have so many babies that if you manage to save even half of them, your tank will become grossly overstocked in about six months.

Hope that helps.

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Lots of fish are black. you should post a link to a picture.
You didnt say what type of tank you have (salt or freshwater)
If it is Freshwater Most likely it is some kind of Ciclid.
Most Freshwater fish are egg scatterers so it is very difficult to tell if they are going to lay but you do need to have a male present to fertilize the eggs.

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Shark !

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probably a black molly and if their stomach gets bigger theats a good sign