Goldfish Care Tips

A Free Monthly Resource For Goldfish Enthusiasts
October 2026
Issue #154

In This Issue
Chlorine Poisoning

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Even small amounts of chlorine or chloramine in untreated water can still seriously harm your Goldfish.

Chlorine Poisoning

Damaged ventral and caudal fins from chlorine burn

I have done it three times in my decades of fishkeeping, most recently this month.

I have been extremely lucky that I have had no deaths, but this last time was a close call.

Chlorine and Chloramine, the Important Difference

Most readers know Chlorine and Chloramine are chemicals added to water supplies to eliminate bacteria and viruses before the water reaches your taps.

The key difference is that you can remove chlorine by letting the water stand for 24-48 hours, boiling it, or vigorously agitating it.

These methods cannot remove chloramine; you must use a water dechlorinator.

Ask your water supplier which chemical they use. Increasingly, chloramine is being used.

My water supplier maintains free chlorine levels between 0.2 and 1.0 mg/L, or 0.2 to 1 ppm (parts per million).

These chlorine levels are extremely low, but 0.05 to 0.10 ppm can still cause serious injury to Goldfish.

Fish Behavior After Poisoning

When Goldfish get a water change, it signals that food should accompany the inflow of fresh water.

During refilling, I watch to make sure my Goldfish are actively looking for food, and I keep doing so after the aquarium or pond is full.

If your fish aren’t foraging for food, start acting lethargic, rise to the surface, or lose color after a water change, suspect the water has Chlorine or Chloramine in it.

Injuries Caused by Chlorine Poisoning

I use the word injuries, but these injuries can be life-threatening.

Chlorine and chloramine burn the delicate gill filaments which can cause asphyxia, the worst outcome. Skin can develop red patches, and fin extremities can get burnt off.

Each fish reacts differently; some show little effect, while others can be badly affected. Smaller fish are affected the worst. My fish are between 75-100 mm (3-4 inches) in body length, which is probably why I had no deaths.

The best treatment method is to put the fish into a mild salt bath until all skin and fin redness has gone.

Keep the water cool and well aerated if the fish are breathing heavily or are staying at the surface.

Remember also that because Chlorine kills bacteria, your filter’s bacterial colony will have been damaged. Clean it if you haven’t already, and test for signs of ammonia after a few days, then daily until nitrate levels start to rise.

Partial Water Changes

You can skip a water conditioner if you're only doing a partial water change. The problem is that you need to know how much free Chlorine or Chloramine is in your water supply, which will dictate how much water you can safely change.

Another danger is water pipe repairs. Contractors dump chlorine into and around repaired pipes to kill pathogens before they turn the water back on. This will obviously cause a massive spike in chlorine levels.

For any partial or complete water changes, I always use a water conditioner, as I suspect even a small partial water change will be either tasted or felt by my fish.

So, What Happened?

My biggest mistake was returning the duckweed to the pond too soon. I should have waited a few minutes to observe the fish.

I couldn’t see the fish clearly, and it started raining, so I skipped a basic safety step I always take when changing water.

A few days later, I noticed damaged fins and a lack of interest in food.

I moved the fish into a mild salt bath, and they're recovering well now.

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