Trouble with corydora

[Colin_T]

Do a 75% water change and gravel clean the substrate every day for a couple of weeks or until we work out what is going on.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

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Do you see the corydoras flashing off of any objects?

Flashing is rubbing on objects in the tank. You will know it if you see it. The fish rubs up against rocks, plants wood or ornaments in the tank. It is usually associated with external protozoan infections.

I do not think this is a protozoan infection and is more likely something in the water irritating the fish.

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If you move established filter media/ material into a new tank, you can normally add fish the following day because you move the beneficial filter bacteria across with the media.

 

[mark4785]

Why wouldn't all the fish be doing the same thing they were all introduced at the same time. I'm gonna do more frequent water changes like recommended. I don't know what you mean by "flashing off objects"

Flashing off of objects just means the fish is approaching an ornament and scraping itself against it to relieve irritation. They wont necessarily all be doing it. If there is any ammonia some may go sluggish, not eat, have inflamed gills while some may scrape themselves.