I went and did a little digging on the lime stone as you asked.. I found that it can create calcium hydroxide or what is sometimes called Slaked Lime.
Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca(OH)2. It is a colorless crystal or white powder and is obtained when calcium oxide (called lime or quicklime) is mixed, or slaked with water.
depending on the levels of lime it can be toxic to the fish. I would be concerned that over time the limestone would dissolve into the solution and burn the fish gills.
RE: Aquaponics Tips #2