05-04-2010, 11:33 AM
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With soft and slightly acidic water, you're ideally suited to almost any of the South American or SE Asian fish: characins (tetras, pencilfish, hatchetfish), rasbora, gourami, catfish, loaches--the list is almost endless. The fish you now have are livebearers, which ironically prefer harder water. They also reproduce like mad, which can be problematical for many aquarists.
I don't like suggesting fish to others, as it should be your aquarium with what you like in it. But given your water, I would definitely see if the store will accept the guppies and swordtail in exchange (most will, after all, they want your continued business) and consider establishing a community of soft water fish. Add some plants, and wow, you could have an incredible aquarium that many others stuck with hard alkaline tap water can only imagine.
Have a look at the photos under my "Aquariums" to see what's possible; these are all soft water, which is what comes out of my tap. And check out our fish profiles section; there are many species with info and photos, and the characin, catfish, cyprinid, anabantid sections in particular are mostly soft water fish. The numbers of fish will depend upon the species; many are shoaling fish that must be in small groups. We can talk about specific numbers if you have some fish in mind.