I had four Zebra Danios in a 10g for just about two years, and a week ago they became sick with what I thought might have been either Columnaris or Saprolegnia. My cycle had suddenly crashed (All my tanks were affected so I think my water chemistry changed); I assume the ammonia spike stressed them out enough to get sick. They had white patches that weren't fuzzy like most photos I found, but did look slightly raised & one had a grayish puffball on its lower jaw. I treated with AQ salt and Triple Sulfa (Supposedly would work for either illness) for six days, and it seemed like they had recovered at first. The next few days they seemed to go from sickly to healthy at random- behavior was off but they looked mostly normal- until yesterday.
Last night there was one fish that was hovering near the filter output and entirely covered in white- like it had been dipped in flour- and this morning all four were covered in white and dead. At least one looked like it had red lesions underneath the white; to be honest I wasn't looking too closely because I was pretty upset. Some research I did last night suggested it might have been Velvet or Costia if it wasn't Columnaris, but I never got a chance to see if it responded to anti-parasite medicine.
I took some photos hoping I could get a post-mortem diagnosis; because I need to re-use the tank and all supplies (No money to replace) I want to be sure I can get it cleaned thoroughly. My plan was to throw out what I can, boil everything that can be boiled- gravel, decor, etc. and then run the emptied-out tank for a few days with a ~10% bleach solution to get at the filter parts before letting it dry out for a week or two. Does this seem like it would be enough to decontaminate the tank & supplies?
Five days ago, my fish looked like this:
This morning, I woke up to this: