beta fish diseases

Question
I have a tank for two betas with a divider down the middle. My newest beta died unexpectedly, so i took him out and bought a new one 2 days later. The newest one kept jumping up and hiting the cover of the tank until he died within 2 days. My first question is, what do you think made him practically kill himself by jumping and hitting his head on the tank cover? Second, he seemed to have stiff-looking fins right before he died, could this have been a disease? because now my red beta who has shared a tank with the first one and the new one is looking grey, and stiff too. Is this a disease that was spread through all three of my betas?  

Answer
Hi Laura;

I would change the water in the tank right away. Their symptoms sound like ammonia toxicity. This is usually from 'new tank syndrome' or excess waste if it isn't a new tank.

How often do you change water in the tank?
Does the tank have a filter?
How long has the tank been set up?

Followups welcome

At Your Service;
Chris Robbins