Why Are My Zebra Danios Dying? 10 Gallon Tank

Hey guys, long time lurker first time poster. I'm worried and confused and figured here would be a good place to ask.

I set up my ten gallon, and after it cycled I introduced five danios, and a few weeks later a betta. This was around 9 months ago, and they've been happy and thriving up until now. Ten days ago, one of the original five died. I double checked my water and had it tested at petsmart too, just to be safe. I added two long finned danios so they can be a happy little school, and they were all doing fine. But this evening another one of them passed away. I don't know if they're just old, or if they're sick, or if I'm doing something wrong. Any ideas?

My water tests at 0 amm, 0 nitrite, <.25 nitrates, and pH around 7.6, at 78F

 

by the sounds of it your betta killed the long finned dinos I don't think they where sick what's your water temp for dinios its best 24 degress and for fighters around 26 they both requare diffrent water conditions hope that helps

 

by the sounds of it your betta killed the long finned dinos I don't think they where sick what's your water temp for dinios its best 24 degress and for fighters around 26 they both requare diffrent water conditions hope that helps

I was a little worried about that, but I really don't think he did? The dead zebras didn't have signs of fin damage or nibbling or anything, and when I watch them, the danios play together, and the betta does his own thing. Why, after nine months of being roomies, would my betta hurt one?

The water temp is 78F, I think that's like 25.5C

 

What are the numbers for ammonia, nitrite and nitrate?

I'm not surewhat killed them but they will be weaker and more susceptible to disease since they are too hot and in a small tank. Zebra danios are super active and need a 20 long minimum. 78f is too hot for them. They need 65-75f with around 70f being ideal. These factors may not kill a fish by themselves but will certainly shorten their lifespan and make them weaker.

Why, after nine months of being roomies, would my betta hurt one?

That's what bettas do. They are not community fish. They almost never go straight in and kill everything. They tolerate it for a while before they snap. Even if a betta never kills another fish it will be stressed by the constant movement (and danios are much too active) and other fish on their territory. 10 gallons is too small to keep bettas with other fish. It doesn't give them enough space to get away from the other fish. Even in a larger tank I would only keep a betta with bottom dwellers such as warm water cories, plecos or kuhlI loaches.

 

well said bottomdweller

 

your welcome @bottomdewller