About a month ago, I purchased a group of 10 juvenile Keyhole Cichlids. They are just under 1" in length. Cute little guys.
This morning, I found two of them dead. Water parameters are fine. No obvious signs of stress or disease in any of the fish (living or dead). The only other tank mate is a 3" bristlenose pleco. I'm not sure what killed them.
When I fed them last night, I gave them some of Omega One's [small sinking] Cichlid Pellets. I've noticed that these pellets swell up to about twice their size once they've absorbed water. I'm wondering...does anyone think it's possible that the pellets could have swelled up inside the little Keyholes, resulting in their death?
Any food can kill if eaten in such a quantity that it blocks the intestine. Ask owners of stupid dogs. "Flat" fish seem especially prone to it. Don't feed so much that a fish gets more than one or 2 bites. Usually the culprit is frozen or freeze dried worms. Directions often say to pre-soak or pre-thaw to avoid problems. Worth a try next time you feed this food.
In a small tank (you don't mention the size) you can also have a deadly ammonia and/or nitrite spike from a single overfeeding and all the parameters will be back to fine before you test.
Another cause of sudden death is a startled head-to-lid impact.
But if the food is the only thing that changed, it is reasonable to suspect it.