You'll never see the two biggest killers of fish and plants lurking in your pond. Chances are, you'll find struggling plants and fish long before you realize these troublesome components are in your water. Unfortunately, for many pond owners, the deadliest threats to their ponds are silent, quick and invisible.
Water quality is the cornerstone of a healthy clear pond. Without healthy water, your fish and plants won't last very long, and your pond will become more of an eyesore than a pleasure. Great water quality all comes back to properly balanced biological filtration. In a properly "cycled" pond, water never contains ammonia or nitrites. However, new ponds and ponds in crisis will exhibit signs of biological imbalance. These signs include ammonia and nitrite concentrations above zero.
High ammonia has three main culprits: in new ponds, with large bacteria die-off or when there's a drastic pH change in your water. In a new pond, high ammonia levels are part of the cycling process. You will see high ammonia until beneficial bacteria are established in your filter and substrate. Until you see ammonia levels drop, your pond is not ready for fish.
Many well-meaning fish keepers kill the beneficial bacteria in their pond by over-cleaning their filters (i.e. rinsing filter pads in tap water or replacing the entire filter pad). Always use pond water to clean your filter pad and change the filter pads gradually- never all at once.
At lower pH levels (under 5.8) ammonia becomes ammonium, a harmless by-product of waste. Left without water changes, ponds become acidic as waste builds up and minerals are depleted. When the pH is brought back up to neutral (i.e. during a large water change), ammonium turns back into ammonia and creates a massive die-off of fish and plants. If you haven't completed a water change in your pond for months, be careful about doing one large water change. Instead, use small water changes over several days to slowly bring your pH back to neutral.
When you have high ammonia levels in your pond (greater than 1 ppm), you'll see your fish gasping at the surface of your pond for air. Often, fish also have enflamed gills and refuse to eat. If you see your fish having these problems in your pond, immediately treat with an ammonia neutralizer or remove the fish to an alternate tank or pond. If all else fails, buy a clean trash can and use this as an alternative tank until you balance your pond's water quality.
High nitrite levels have two culprits: in new ponds and when there's a bacteria die-off. If you have a new pond, you will see an ammonia spike followed by a nitrite spike. Eventually, nitrifying bacteria will establish themselves in your filter to turn these nitrites to nitrates. Nitrites are very toxic to fish, while nitrates are harmless. Like ammonia, you can have a nitrite spike if you over-clean your filters in tap water or replace all your filter pads at the same time.
If you have high nitrite levels (above .5 ppm), you will see your fish gasping at the surface of the water. Their gills will turn brown (instead of red) and they will usually look much darker than normal (especially koi). If you notice these symptoms, immediately add aquarium salt per the dosing instructions on the package! Aquarium salt will save your fish's lives by neutralizing the ion that attacks your fish's blood when you have high nitrite levels. After you have added aquarium salt, you can control the nitrite levels with water changes until the nitrite levels drop again.