Every time I do a water change, my corydoras and tetras start acting weird within a few hours - hiding, gasping at the surface. I use the same tap water conditioner I've always used, but this started happening recently. Could something in my water supply have changed?
It's probably not your water supply that changed - it's more likely something about how you're doing the water changes now. Check if the new water temperature matches your tank (even a few degrees off stresses fish badly), and whether you're changing a much larger percentage than before. Also test your conditioner bottle itself - if it's old or got exposed to heat, it might not be working properly anymore, or you could accidentally be overdosing it. Try doing a smaller change with water that's been sitting out to reach room temperature, and see if the fish react differently.
Test your tap water directly - call your local water utility and ask if they've recently changed treatment chemicals, added chloramine instead of just chlorine, or flushed the lines, because conditioners handle chlorine fine but some don't neutralize chloramine quickly enough. If that checks out, the culprit is usually temperature shock (adding water that's even 5 - 6 degrees off stresses corydoras badly), so match your tap water temp to the tank before adding it in. Also verify you're not accidentally using a higher dose of conditioner than before, and do a partial water change on a smaller tank or bucket first to see if the same reaction happens - that'll tell you if it's the water itself or something about the change process.
Start by checking how long you're letting the conditioner sit in the new water before it goes in the tank. Some folks don't realize that water conditioners need a few minutes to actually work - if you're pouring treated water straight in, the chemical might not have bonded with chlorine/chloramine yet, especially if your water utility switched to chloramine recently. That's a sneaky one because it seems like nothing changed on your end.
Beyond temperature matching (which the others covered), there's something practical worth trying: do a smaller water change next time, like 15-20% instead of whatever you normally do, and space it out over an hour by adding the water slowly in stages. This gives the fish time to acclimate gradually instead of getting shocked, and it also gives you a chance to see exactly when they start acting stressed. If they're fine with a slow small change but freak out with your usual amount, you've narrowed down whether it's the volume, the speed, or something actually wrong with the water itself.
One more thing - if you've recently switched where you're storing your water or started using a new bucket or container, that could be an issue. Old plastic buckets can leach stuff, or if you're using something that had chemicals in it before, that'll cause problems fast. Keep your water change supplies separate and clearly labeled.
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