If several fish suddenly gasp, flash or lose their balance after a water change, check how the new water was treated and the equipment first. Changes in chlorine, chloramine, temperature and pH can harm fish, and they can also interact with water quality problems that were already there. Improve aeration first, stop any more unverified new water from going in, and then track down the source of exposure.[1]
What to do first when something goes wrong
Make sure aeration, filtration and water flow are working, test the temperature and pH of both the tank water and the new water, and test ammonia and nitrite in the tank. If you have suitable test kits, test free chlorine and total chlorine in both the new water and the tank water right away. Write down how much water you changed, where it came from, which products you used and how much you added.
If you need to change in safe water or move the fish, first confirm dechlorination, water quality, temperature and aeration in the new system. Don't add more of the same batch of water that hasn't been fully treated; how much water to change and how fast should be judged by the contaminant and the condition of the fish, not by a fixed percentage.
If fish are still clearly off balance, keep breathing rapidly or are dying one after another, contact a veterinarian at the same time. As long as it doesn't delay emergency care, keep water samples from before and after the change and the product information to help work out the cause.[2][3]
Why both chlorine and chloramine matter
Tap water may be disinfected with chlorine or chloramine. Chloramine is a chlorine-based disinfectant stabilized with ammonia; you can't assume every water supply area uses the same treatment, so confirm with appropriate testing.[1][4][6] Taiwan Water Corporation, for example, states that it disinfects with chlorine and maintains an appropriate level of residual chlorine, and its explanation doesn't mention chloramine; the chloramine information in this article comes mainly from US sources, so check with your own water utility about local practice.[7]
| Test | What to look for |
|---|---|
| Free chlorine | Mainly measures active chlorine that isn't bound to ammonia. |
| Total chlorine | Includes free chlorine plus combined chlorine such as chloramine; check the test kit instructions. |
| Only free chlorine is negative | Can't rule out chloramine on its own. |
| Chlorine reads negative after the exposure | Chlorine may already have reacted or dissipated; earlier poisoning can't be fully ruled out. |
No chlorine or chloramine should be detectable in a system with fish. These readings change easily over time, so testing on the spot is more useful. A single normal reading taken afterward can't overturn the water change timeline and other test results.[1]
What to check on a dechlorinator
Check whether the label says it handles chloramine and how it deals with the related ammonia by-product, and check the volume of water to treat against the label.[1] Products differ in concentration and use, so don't apply another brand's number of drops.
Removing the chlorine part of chloramine can release ammonia. A mature biological filter can help process it, but a new tank or a damaged filter bed may not be able to handle the added load, so retest ammonia; "I already added dechlorinator" doesn't mean every water quality parameter is normal.[1][5][6]
Letting water stand, aerating it or waiting a set amount of time can't be relied on as chloramine treatment for every water source. The most reliable approach is to know your water source, treat it with a suitable product according to its label, and confirm with appropriate testing.[1][4][6]
Besides chlorine, what other changes to check
Compare temperature and pH before and after the water change.[1][2] Also look back at the heater, the mixing ratio, contamination from cleaning products or tools, and whether you cleaned the filter media heavily at the same time; avoid changing several conditions at once, or the source becomes harder to trace.
Pay particular attention when total ammonia has already built up in a slightly acidic tank. When new water suddenly raises the pH, the proportion of more toxic un-ionized ammonia can increase; ammonia readings have to be interpreted together with temperature and pH, not just by whether total ammonia went down after the change.[5]
If the problem didn't appear right away, gill disease or other illnesses still need to be considered. Noticing a problem after a water change only shows a link in timing; it doesn't automatically prove that a particular product or water source caused it.
How to make the next water change easier to trace
Keep a consistent water source, containers, treatment routine and water quality records; when you change water source or product, check the label and test first. Keep cleaning products away from aquarium equipment. Maintain a stable environment suited to the species.[2]
Watch breathing, appetite and swimming before and after water changes. If there was already a water quality problem, keep retesting after the change until the filter can handle the load reliably.
Frequently asked questions
Is adding more dechlorinator always safer?
Don't add more on a whim. First check the ingredients, what the product covers and the label dose; if the fish are seriously ill, ask a veterinarian to help plan treatment.
If fish die after a water change, is it always chlorine poisoning?
No. Chlorine, temperature, pH, ammonia and other contaminants all need to be assessed against the timeline and test results.[1]
Further reading
- What to Do When Your Fish Is Sick – Reading Aquarium Fish Symptoms and What to Do First
- Ammonia and Nitrite Poisoning – Fish Gasping at the Surface and How to Read Water Test Kits
- Popeye and Cloudy Eyes in Fish – Sorting Out Eye Injuries, Water Quality and Infection
References
- Francis-Floyd R, Yanong RPE, Petty BD. Environmental Diseases of Aquatic Animals in Aquatic Systems. Merck Veterinary Manual, revised 2023.
- Francis-Floyd R, Stilwell NK, Yanong RPE. Introduction to Fish Health Management. UF/IFAS, CIR921/FA004, 1990 (revised 2025).
- Stilwell NK, Yanong RPE, Francis-Floyd R. Submission of Fish for Diagnostic Evaluation. UF/IFAS, FA055, 2001 (revised 2025).
- Crosby TC, Hill JE, Martinez CV, Watson CA, Pouder DB, Yanong RPE. On-Farm Transport of Ornamental Fish. UF/IFAS, FA119, 2005 (revised 2021).
- Francis-Floyd R, Watson C, Petty D, Pouder DB. Ammonia in Aquatic Systems. UF/IFAS, FA031, 1990 (revised 2022).
- Pinellas County Utilities. Frequently Asked Questions About Chloramines, accessed October 2026.
- Taiwan Water Corporation. Frequently Asked Questions (in Chinese). Taiwan Water Corporation website, 2026.