Cory cat died

[London78]

Hello, would anyone know why my Cory cat just died. I’ve had it a week. I got that and 2 tetras as my first fish to set up the tank. I’ve done 2 part water changes this week. 25%. It had been starting very still for a couple of days. Hardly moving. This afternoon I noticed it was on its side, again not moving. I did a water change again. And also left a cooked pea for it but tonight it just wasn’t breathing or moving. Could it be the fact I got it by itself? The tank has some plants in and hiding places. It just doesn’t seem to have been happy. I’ve checked the water. Ph level fine and nitrate level fine too. Any suggestions would be appreciated. Thank you

 

[Essjay]

What are the readings for ammonia and nitrite? If the tank was not cycled before you put the fish in the tank, you may well have some ammonia and/or nitrite in the water despite having live plants.

Being in a group that's too small doesn't usually kill fish quickly - it causes stress which reduces the immune system to the fish get sick more easily.

 

[PlasticGalaxy]

Very sorry for your loss.
It could be that the cory had a disease when you got it. Alternatively, like @Essjay said, it could be due to ammonia or nitrite.
Either way, in the future you should always get groups of at least three corys and quarantine new fish for anywhere upwards of two weeks before putting them in a tank.
What tetras did you get? Tetras are schooling fish, so they need to be in groups of around eight as a minimum.