Sadly, Azul has passed away . He had some kind of illness so I will need to sanitize the tank and everything in it before getting another betta. I have heard using hot water and vinegar is a good way to go, but I'm not sure if that's true? What's the best way to sanitize the tank so that my new fish will not get sick?
I really wish I could help answer this, but I'm not really sure myself!
I know not to use bleach or any other chemicals...
What I do is scrub really hard with some soap, but rinse it out really REALLY good with scalding hot water. And I make sure there's no chemical smells left over.
Probably not very helpful, but this is what I do and so far it has worked. :/ I think the vinegar idea is better.
With my acrylic tank I used a 1 part bleach 9 parts water ratio, let the tank soak and washed with an aquarium approved sponge (don't use a sponge you get from the grocery store, some say not for aquarium use), rinsed well. Then put it in the dishwasher and ran it twice with no soap. Took the tank out and set it outside. Set it outside for at LEAST 6 hours. Even for a couple days if you can help it. Brought it back inside, twice more in the dishwasher and I was good to go.
With a glass tank, don't put it in the dishwasher. I do believe the bleach method would work though. Just rinse, rinse, rinse, rinse, rinse, rinse and when you think you've rinsed enough, rinse some more.
As for substrate, I would throw it away and buy new stuff if you can. Cleaning sand is damn near impossible and gravel isn't much better. Decorations you can usually boil or do the dishwasher route. Or, throw away and get new stuff. Either is your choice.
Sorry some of us do have a life, career, school, etc and can't come at every beck and call. Hope we did help.