Posted by Chris Lewis ·
A tank that's been stable for years, no new fish, everyone else acting normal, and one fish is just gone. Here's a short checklist before you worry about the rest.
1. Temperature. This one gets missed a lot. Many common fish, including most corydoras, prefer the low-to-mid 70s °F. Water held in the mid-80s speeds up metabolism, lowers oxygen, and shortens lifespan over time. Summer heat can push a tank up without anyone noticing. Check the species page for your fish: for example bronze, panda and sterbai corydoras have different ranges.
2. Oxygen. Warm water holds less oxygen. Fish gasping at the surface, or corydoras darting up for air far more than usual, are signs. An air stone is cheap insurance.
3. Water numbers. Test even if you think it's fine: ammonia, nitrite, nitrate, pH. A dead fish hidden behind the scape can spike ammonia on its own. Water Check will tell you if anything's off.
4. Tap water changes. Cities change treatment, especially seasonally, and some switch from chlorine to chloramine. If you don't use a dechlorinator, start. It costs pennies and removes a real risk.
5. Age and luck. Sometimes one fish dies of something internal and nothing else happens. If the rest of the group eats, swims and looks normal for the next week, that's the most likely answer.
Watch for the next week: appetite, breathing rate, clamped fins, spots or patches. If a second fish shows signs, it's time to look for a disease rather than a one-off.