Fallen leaves can cause serious damage to a pond as they decay and add to amount of ammonia and other chemicals in the pond. The chemical changes induced by decomposition can disrupt the pH balance and worsen the effect of toxic gases produced from that decomposition. When temperatures dip well below freezing, the surface of a pond will become largely frozen over, inhibiting the normal gas exchanges that protect fish during the rest of year. Without the opportunity for gas exchange, your fish will die very quickly. You can help prevent this sad event by performing a thorough fall cleanout before cold weather strikes. Remove any organic matter that's floating on the surface or has sunken to the bottom of your pond. Trim cold-hardy plants and move them to the deepest part of the pond and remove the others. Once you've finished your cleanout, consider adding a leaf-net to your pond to keep out any that may blow in as the season progresses.
Another important stragegy to manage the risks of toxic gas buildup is to install a gentle aerator in a shallow part of your pond that is not sealed by ice. Your goal is not to mix the water throughout the pond, but to encourage gas exchange and keep at least some dissoved oxygen in the water.
