silver dollar attack
 

I bought 2silver dollar fish I put them in my tank and the fish attacked them I have a blue gurami and some koi ,a shark ,a dragon eel ,danuao ,guppy and gold fish
The gurami and the gold fish were the attackers the killed one before I could pettition the tank now the gurami keeps bumping the pettion trying to get to the silver dollar. Why?

What size tank is this? What are the water perimeters? Also when you say "Dragon eel" what is it's real species name?

70 gal. Don't know what he is he wasn't the problem it was the gurami and the gold fish

You have a mix of fish that are not compatible. First, goldfish and koi should never be in the same tank as any tropicals. Second, gourami can be aggressive; without knowing the species (there are many species of gourami) it is possible you have a male and it views the Silver Dollar as another "gourami" because of their basic shape, and the gourami is making it clear he doesn't want them around his territory.

Some fish need to be in a group, some in pairs or singly.

Have a read of the profile for the species you have, under the second heading from the left in the blue bar across the top of the page. They will explain the needs of these various fish.

Byron.

Why did the goldfish attack
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Possibly due to stress. When non-compatible fish are combined, it causes them stress, and this often leads to increased aggression even from peaceful fish. They are frustrated.

You have a fish and you don't really know what it is? How can you do research on it if you don't know? If it is what I think it might be, it could be a stressor on the other fish.

I agree with Byron. Stress can cause normally placid fish to lash out. Goldfish are not aggressive fish and shouldn't be behaving in this manner. The tank size is actually fine for goldfish (but not koi... they need 500+ gallons per fish), but the tank mates are not.

So simply put, the goldfish attacked because something is wrong with your setup.