The thing with many illnesses/diseases/pathogens... it's that they're often unpredictable. Some virulent strains of bacteria and viruses can lay dormant for months, even in dry conditions. A quarantine period of 4-6 weeks is a reasonable/acceptable time frame... but does give us a 100% guarantee that we're not introducing illness into the tank... just a "safer gamble" we're taking. As well... some of the same illnesses present different symptoms depending on the fish and their immunity levels, genetics and environment.

Most "run of the mill" pathogens show themselves within a 4-6 week time frame (including from a stressor, such as shipping, rehoming, drastic temp/pH fluctuations). It is said that the bacteria that causes flex (columnaris) or the ich and velvet parasites, for example, are ubiquitous (ever present)... so keeping the stressors down, and ensuring a healthy environment (water, food, etc.) keeps these from affecting your fish.

Unfortunately, there's no real good answer here... despite being an excellent fish keeper, doing our best to prevent things from going on... things still happen and THAT is frustrating, I know! All we can do is the best we can at preventing something from popping up... by ensuring their environment is healthy and reduce the stressors as much as humanly possible.

If you don't suspect fish tb, I would still keep your tank under "quarantine" for 4-6 weeks past the last death or sign of illness before introducing new fish... and concurrently (or afterwards) quarantine any new fish entering the system. Still not a guarantee... but short of quarantining for possibly a year (which isn't reasonable in most instances)... hopefully, you'll be okay.

Keep us posted on what's going on with your remaining fish! Join us in our various forums, as well, to talk about your fish and tanks! If you haven't already found our Freshwater Forum... stop by for a visit!

Michele