My betta just died...

Question
I'm broken hearted.  I've had Lyndon for almost three years now.  He got very sick a year ago and I think I bought every antibiotic you can buy for bettas and he got healthy and strong and was doing great until about two days ago when I noticed he wasn't eating as well and wasn't as active.  Just this morning I thought his color didn't look good.  I change his water every Sunday morning, and when I went to change him, he was gone.  The water temp was 78.9 and the pH was 6.5.  I do a complete water change and add aquarium salt and Stress Coat every time.  What could have gone wrong and do you think there was anything I could have done?  It just happened so quickly.  I'll eventually get another betta but am afraid of disease in the tank.  I threw away his two silk plants, but is it okay to re-use the tank (2.5 gallons), stones and heater?  I feel like I somehow let him down and wouldn't want to let it happen again.  Thank you.

Answer
Couldve been a secondary infection. A lot of times, when treating for one issue, a sencondary infection arises. It is ok to use what you had. Just empty it out, clean t and let it dry before setting it up again. I'm sorry I couldn't get to u sooner......................