Here are some potential reasons:
* Water Parameter Changes: Even if you carefully match the temperature and chemistry of the new water, there can be subtle changes that affect your fish. These could include changes in pH, chlorine levels, or dissolved oxygen levels.
* Temperature Shock: If the new water is a different temperature than the old water, it can cause temperature shock, leading to lethargy and even illness.
* Stress: Water changes, even when done carefully, can be stressful for fish. This stress can manifest as lethargy and loss of appetite.
* Chemicals: Even if you use a dechlorinator, small amounts of chlorine or other chemicals can remain in the new water, causing stress and lethargy.
* Uneaten Food: Leftover food from the previous water can decompose and release toxins, making your fish lethargic.
* Disease: While less likely, illness can also cause lethargy, especially if the fish is already stressed from the water change.
Here are some things you can do to help your fish:
* Monitor the Water: Use a water testing kit to check the pH, ammonia, nitrite, and nitrates levels.
* Maintain Stable Temperature: Ensure the new water is the same temperature as the old water.
* Acclimate Slowly: Don't dump the new water directly into the tank. Acclimate the fish slowly by adding small amounts of new water over time.
* Use a Dechlorinator: Always use a dechlorinator to remove chlorine and chloramines from the tap water.
* Clean the Tank Thoroughly: Make sure to clean the tank thoroughly during the water change, including the gravel and filter media.
* Offer Fresh Food: Offer your fish fresh food, and remove any uneaten food after a short time.
* Observe for Other Signs: Observe your fish for other signs of illness, such as loss of appetite, discoloration, or unusual behavior.
If your fish continues to be lethargic for more than a day or two, contact your veterinarian or a fish expert for help.