03-27-2007, 09:17 PM
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You can use calcium based substrate with the proper kind of freshwater fish. In addition to the obvious African Rift Lake Cichlids, most of the Gulf of Mexico Littoral, including the Everglades, sits on a big limestone shelf - fossil coral, essentially. So any of the fish from that area thrive in hard water - Mollies, Guppies, Platies, Swordtails, Firemouth Cichlids, Texas Cichlids, Pupfish, Florida Flag Fish, Pygmy Sunfish, Violet Gobies, to name a few. And you won't need to salt the water, as all of them are evolved to regulate their osmotic pressure equally well with calcium or sodium. The only thing I have had trouble finding for that biotope is a catfish that doesn't get enormous or need straight up brackish water. Blind Cave Tetras also come from a similar situation - limestone caves.
So you can use it with freswater fish, but you just have to be more selective. Also, check what plants you can use carefully. Basically, if it can handle brackish, it can handle hard water. Vals are the typical hardwater plant, but hornwort, Elodea, Java Moss, Java Fern, and some Anubias and Crypt species do really well too.