Hi Cossack1977
I'm so sorry about your fish.
I think Rose has the right answer. Flexibacter Columnaris is a bacterial infection that can devastate an aquarium, and it can happen very quickly. That's why I asked if you saw any finrot which is one of the symptoms.
This is a bacteria that is present in all tanks. It usually affects fish when they have been under stress of some kind which weakens their immune systems. This can happen through a rapid rise in temperature, low oxygen content in the water, high nitrites, uneaten food, insufficient or infrequent water changes, overstocking, even aggression by tankmates.
Some strains of this bacteria work quickly, others slowly. I bought 5 wild caught corys almost a month ago and they had the disease when I got them. Despite antibiotics, I am still unable to cure them; all I can do is keep it from getting worse. Tonight I started giving them salt water baths, which is something I hate to do to a cory. Some strains can work so quickly that there is not even enough time for the fish to show symptoms.
I don't think you need to clean with anything more than table salt, if you take your aquarium apart. You might want to boil your gravel. Chlorine bleach is a good disinfectant, but be sure to rinse with water that has a liberal amount of dechlorinator in it and let it air dry, if you us it. Avoid any other type of household cleaner. But, do clean out your filter and also your nets and siphon and anything else used for the fish.
Good luck and please let us know what you do with your tank in the future.