Like I said before, my uncle is pretty sure at this point that it was poisoning due to the metal his grandson dumped in the tank. (Yes, his grandson was reprimanded even though he claimed it was an accident and he is seeing how what he did is dangerous to the fish)
i used to have danios, had them for about a year and woke up and BOTH danios had this weird curved spine. I didnt know what it was(i was inexperienced at the time) and removed the fish drained the tank and disinfected everything.
thanks for helping me understand what killed them and why they had bent spines