I wouldn't give antibiotics to a fish that isn't sick. Every time you do, you run the risk of creating antibiotic resistant strains of bacteria. Just take extra precautions to keep everything that touches their water as sterile as possible.
This happened to me once in my divided tank, actually, it was horrible. One fish suddenly dropped dead and then slowly over the course of the week I lost the other two, one that I'd had for four years and one that I'd had for less than a month. I was sad to lose one I'd had so long, and just as sad that the other never got the chance to live a full life. The antibiotics did nothing, so I assumed it was viral or some kind of super strain. I totally empathize with you.
All I could do in the end was sterilize everything with potassium permanganate and start over.
I recommend reading this article on the risks and the proper usage of antibiotics: http://edis.ifas.ufl.edu/fa084 it will give you a better idea of how antibiotics work and why "shotgunning" isn't always an appropriate response. I hope things start going better for you.
Thanks Adastra. That article helped a lot. It's so tough when you don't know what you're fighting. I mean, if I could see something I could do something. I won't give the others any tetracycline. I'll just keep them well fed, water changed and all. I'm definitely going to be more careful about my tools. I replaced everything but my bucket and my plastic bottles. I just gave the bucket a good cleaning with a 1% bleach solution. I'm going to get new plastic gallon bottles too.
Senor was the littlest betta I had. The only other I would really worry about is Whiskey. He's got a tumor to begin with and he's thin from it. If anyone else goes it will likely be him.
Senor is still fighting. He reacts when I come in the room and goes up for air, sometimes upside down. :( I feel so helpless. In the year and a half I've kept these guys these are my first losses. He's just a lil guy and I feel like I failed him somehow.
I was thinking it could be in the water only because the water in our valley is being tested for Legionnaires disease which is shocking because im more worried about what I will do about water changes rather than my own health haha.