04-20-2010, 09:53 AM
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I second 1077, mollies in my experience do best on their own. They make a lovely display, a group of black mollies (or the others) in a 20-30g tank planted with Vallisneria (corkscrew), pebble-type of substrate, with the water moderately hard and basic (pH high 7's) is very impressive. A nice Central American stream setup.
Perhaps your store will take back the remaining mollies; with neons I would not raise hardness or pH.
And I second Johnny, nitrate at 5 is not going to cause problems for fish. Most consider 20 ppm or lower the normal nitrate range, and while many fish can go above this it is preferable to keep nitrates below 20 ppm. Plants will do this, nitrates in a well-planted tank are rarely above 10 ppm and often zero, or if no plants then regular weekly partial water changes do the trick, provided you don't have nitrate in your tap water.