You did all right for the first year. Most people kill their fish in the first 2 weeks so you're ahead of them
Bettas don't live that long and usually die after 2 or 3 years, sometimes they last longer (5 or 6 years) but that is uncommon. But it depends on how old the fish was when you got it, and that is virtually impossible to know unless you bought it from a breeder. But chances are the fish is middle aged and its immune system is not as good as it use to be and it just got sick.
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You can do a 100% water change each day as long as the new water is free of chlorine/ chloramine before it is added to the tank, and as long as it has a similar temperature and chemistry (pH & GH) to the tank water. If you are using the same water supply for water changes, then it should have the same chemistry.
Normally tho a 75-80% water change done each day for a week will get rid of most things in the water. This lets the fish sit in a bit of water at the bottom of the tank but still dilutes and removes most of the chemicals and disease organisms in the water. Again tho, any new water has to be free of chlorine/ chloramine before it is added to the tank.
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Water changes don't normally affect the beneficial filter bacteria that like to live on hard surfaces like the glass, gravel and filter materials. There will be a small amount of beneficial bacteria in the water but the amount is tiny and not worth worrying about. Most of the good bacteria will be living in and on the filter materials.
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Broad spectrum medications are made by most fish companies but you try to find one that treats bacteria, fungus and protozoan infections. They usually contain Methylene Blue, Malachite Green & Formalin (Formaldehyde). Some won't have Methylene Blue but most contain Malachite Green and Formaldehyde or a form of Formaldehyde.
Methylene Blue will wipe out filter bacteria and can stain the silicon (glue holding the glass tank together) blue but it does not cause any damage to the silicon. It just makes it blue. But you need to monitor ammonia and nitrite if using this.
Malachite Green is carcinogenic so wash your hands with soapy water after using medications or working in the tank.
Formaldehyde is a preservative and can cause health issues too.
Fortunately there are only small amounts of these chemicals going into the tank and these get diluted out quickly when you do water changes. But regular exposure or prolonged exposure can be harmful so handle with care and wash up after using them.
*NB* Keep medications and test kits in a cool dry place away from children and animals.
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Furan 2 will treat more types of bacteria so covers a wider range or possible targets and is generally considered safer than Kanamycin, so it is probably the better choice out of the 2 medications. You will still have to monitor the ammonia and nitrite levels after using it because it will probably wipe out the filter bacteria.
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Melafix and Pimafix (not sure about Bettafix because I have never used it) contain Tea Tree oil and makes your aquarium smell like a Eucalyptus tree after it has rained, but in my experience it doesn't help with fish diseases. I found cleaning the tank conditions up (big daily water changes, gravel cleaning the substrate, cleaning the inside of the glass and cleaning the filter) make more of a difference than them.
Bettafix, fixes your Betta but might also kill it. Therefore it fixes the problem the Betta had but creates a different problem, which fish to buy as a replacement for the one you killed
