Hi Bea/\/
I'm so sorry to learn that you lost your guppy.
But let me explain what I think happened so you can take care of the ones you have and prevent the same thing from happening again.
Columnaris Flexibacter is a bacteria that causes a variety of symptoms in fish and is very common. This bacteria can always be found in a tank, but if the fish are otherwise healthy, it causes no harm. But certain things can stress a fish and make it more susceptible to infection.
Some of these stressors are overcrowded conditions, uneaten food in the tank, high nitrites, low oxygen content in the water, sudden rise in temperature, infrequent water changes or aggressive fish in the tank.
When you turned off the filter, you also cut off the water circulation that helped the water pick up oxygen from the surface. This is a condition that could cause the Columnaris bacteria to suddenly increase, while your beneficial bacteria died, especially if the temperature is high.
Columnaris can act very quickly, once it attacks a fish, or it can develop slowly. It can look like a fungusy growth on a fish's mouth or body or it can look like finrot. It can even kill a fish with no symptoms.
Right now I suggest you give your tank a good bottom cleaning and water change to get it back to normal, and watch your other fish for symptoms for a week or so before adding any new fish.
Your cleaning schedule is good and under normal circumstances would probably have gone a long way toward keeping your fish healthy.
Don't worry, your tank will indeed get back to normal. It's just so much better to be cautious once you know you have had a problem.
