If you have not had it done get your pond bottom soil tested and learn all you can about water quality and the food web (base).

http://aquanic.org/publicat/state/il-in/as-503.htm

http://srac.tamu.edu/index.cfm?catid=25

http://srac.tamu.edu/tmppdfs/13452410-464fs.pdf?CFID=13452410&CFTOKEN=70431263