Hello
friedmihunSorry to know about your betta.
It's a learning process so you'll know what you did wrong and do better next time.
Okay, I see you got a halfmoon doubletail betta. The thing about these fancy tail bettas is that they are susceptible to diseases because they go through a lot of inbreeding to get that fancy tail. So to take care of these fancy tail bettas you'll need to really take extra care in its water parameters and maintenance.
Personally I prefer plakats which are very hardy and not susceptible to disease. You might want to consider a plakat for your next betta. Occasionally I rescue poor veiltails from LFS too if I have an empty spare tank.
Veiltails are damn hardy too.
To answer your questions:
1. Your daily water change routine is fine to start with and you don't need a thermostat. Once your tank is cycled, you actually don't have to change your water that often. Since your betta just died, do a thorough clean of the tank, put the gravel back, fill it with water and let it sit for a few days. Also, if you want to cycle your tank quickly, I recommend you buy a bottle of Nutrafin Cycle (some LFS sell them). It really helps when starting up a new tank and you can use it for regular maintenance too when you change water.
That piece of wood in the tank, is that newly bought as well? New wood will leech tannins into the water. I don't know if they are harmful to bettas but perhaps you should remove it until your tank is properly cycled.
2. Yeah, sometimes the betta might be carrying the virus and maybe poor water conditions triggered it. Hard to tell and I'm not an expert on this.
3. Sorry, can't help you with this one. Actually never encountered velvet disease before.
4. Yes, multiple diseases can set in when the fish is sick. The fin rot was probably caused by the velvet disease. Again, I'm not sure if you can mix medicines together. Maybe someone else can help you here.
5. You only use aquarium salt to treat diseases. If your fish is fine, then you shouldn't use it. For 2.5 litres, I think you only need very little - maybe 1/4 teaspoon? Table salt contains additives so it's not recommended for aquarium fish.
You can start cycling your tank again from today and by the weekend you can go look for a new betta. Haha.
Yes i've learned so much and from my first betta's passing, I hope I'll do better next time.
I've already cleaned my tank and cycled it. Yes that is a new wood bought from a LFS. I pre-soaked it before i put it into my tank previously, so i hope that's fine.
I've finally found a medication for velvet called "Ocean free- Super slime, white spot & Velvet away". Haven't personally used it yet as my betta has just passed, but good to know/keep an emergency kit around in case my future betta is sick
Yeah I'll prolly get another betta soon and I hope this time its gonna be alright. I kinda like halfmoons with their fancy tails but i guess amateurs like me should try plakats as you have suggested