I have a new set up (about 65l or 15 gallons). I conditioned the water without fish for 3 weeks. Then about three weeks ago I introduced 3 platys (1M and 2F). The fish appeared happy if a little shy. Then the male started swimming up and down very quickly. Within an hour he was dead at the bottom of the tank. That was a week ago. Today one of the females started showing the same behaviour. And the result was the same. Dead at the bottom of the tank. The second female is now showing the same ditzy behaviour. The only clue that they were sick was the change in behaviour. I didn't see any other signs. Ph has been in the range 7.0 to 7.5. I have a piece of wood/root in the tank that is releasing tannin and making the water a little brown. My supplier assures me that this is harmless. My best guess from reading this website is a nitrogen problem but the symptoms don't seem to match very closely. Any tips as to cause and solution?