05-27-2013, 11:13 AM
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First on the fish itself, you need to separate it to another tank, though it may not recover depending upon what damage has been done. If this is not possible, euthanizing the fish is best.
The reason this is happening, and will continue to happen, is the selection of fish. You have too many species, and not enough numbers of some of them, and in too small a space. Some of the fish you have are quite aggressive: the Tiger Barb and Green Tiger Barb (the same species), the Rainbow Shark. A 20g tank is not sufficient space for any of these. Read more in our profiles, click the shaded name.
I don't know what the Green electric tetra might be, but it sounds as if it is trouble too. The gourami will likely be the next fish attacked, and the rosy barb won't be far behind.