dying mollies and platies

Question
2 foot fesh water tank at 25*C. Normal chemistry.
cannot keep mollies or platies or swords for longer than about 4 months.  Can show fast erratic swimming from end to end of tank. otherwise fish appear normal.

Answer
Hi Andrew;

What other fish do you have with them?
How often do you make water changes and vacuum the gravel?
What kinds of foods do you feed?
What is the water temperature?
What kind of filter?

Let me know as soon as you can.....

Chris Robbins