02-12-2013, 08:03 PM
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Originally Posted by Byron
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.