Hello, I am the son of Netti. Sadly, currently I am the betta killer :(
Well, we are frustrated with the situation (of course) and I am quite sad as the current fish (Well, pet store they would try to fix him with aquarium/epsom salts and call if he recovers) was quite personable. I was able to play with him, he would come to my voice, and etc.
I asked the pet store guy and he said that they had no problems with any of the other fish, so it had to be something we're doing. I took the store credit they offered (will give it back if they fix our poor fishy), but need a break from killing fish (it's hard work after all!).
The zebra danios we'll try in the tank will hopefully survive, dictating that it is some sort of... curse?
Thanks to everyone here (especially my mum) for helping me out as much as possible, despite the most probably outcome.
If you're wondering, my avatar is currently of when we first got this fish. Hopefully it'll give him some good luck while at the pet store.