Nitrifying bacteria in a pond is very good for fish and plants, if for any reason Chlorine or chloramines enter the pond, (maybe by topping up your pond with tap water) it kills this friendly bacteria, it can be harmful to fish and may burn or even kill any aquatic plants.
Plenty of water circulation will dissipate the chlorine normally within one or two days, chloramines take a lot longer.
Always try to use rain water for top ups, although you still need to be careful as you might not know what toxins the water is collecting on its way to your rain butt.
A solution called E.D.T.A. (Ethylene Diamine Tetra Acidic Acid) is used to remove any copper or iron from your pond, i suggest before using it, read any instructions carefully, if you overuse it there could be a dramatic drop in your PH levels which cause the fish to go into PH shock, they then get stressed, it can also cause diseases to break out.
Any white foam that appears when water is entering your pond from a waterfall or fountain etc shows a high level of dissolved organic compounds.
If this happens try and do some partial water changes or use an additive that can control the high organic load, I have used aqua gold in the past, but search around, you will be able to find a good recommended product that will get the job done.
Balance is everything when keeping a healthy pond, if you set it up right you will enjoy the benefits for years to come.
Return from chlorine-chloramine to water quality