Cardinals dying?

Last week I bought 5 cardinals. They seemed to be doing really good. Untill yesterday I couldn't find one. I have a male betta and a large decoration rock where they can hide in. I noticed one of the cardinals missing and my betta has been in the rock for a while. I removed the large rock and one of the dead cardinals fell out.. So that makes 4. This morning I only seen 3 cardinal tetras! I found a body stuck to the filter input, and my betta was right next to it.

Could my betta kill those cardinal tetras? He's really shy and usually does not hurt anyone.

 

He could be killing them at night. What size is your tank?
Adding more hiding spots might help.

 

He could be killing them at night. What size is your tank?
Adding more hiding spots might help.

Theres lots of hiding spots, its a 15 gallon tank and I have large plants on the back and rocks/caves. I lost the first one already during the day but he might be killing them. I put him in the breeder box to see if the rest makes it

 

When stocking the tank, did you add the betta first or last? How you do it can cause some aggression in bettas as well.

 

When stocking the tank, did you add the betta first or last? How you do it can cause some aggression in bettas as well.

The betta was in before the cardinals got in. About a week after. I've seen no aggression from the betta yet.

 

It safer to have the cardinals in first. That way he's moving into their territory and not the other way around. To fix this you can try moving decorations around to mess up his territory so he has to start again.
How long ago did you add the cardinals?
What are your water parameters/ water change schedule?

 

It safer to have the cardinals in first. That way he's moving into their territory and not the other way around. To fix this you can try moving decorations around to mess up his territory so he has to start again.
How long ago did you add the cardinals?
What are your water parameters/ water change schedule?

I got them on wednesday. I do water changes every week around 30%. Idk the parameters since i have no tests. Changing the decor will be a hassle since i just did an aquascape

 

It's always possible that you got a bad batch. Or the store just got them in and moving from being shipped to their tank to your tank was too stressful.

 

You may just have a betta that wont tolerate tank mates. It happens, some are more aggressive than others.