what could have happened to my fish?

Question
hi, my names lisa i a writing to ask for you opinion/advice on what has recently happened to my fish.
i have a 40 gallon tank that had 4 fish in 2 goldfish 1 black moor and a comet. The comet has never looked a healthy fish but has always fed and sempt fine. on monday we noticed it looking particularly ill ( not swimming properly ) and knew that it was probably going to die. At the same time we had noticed a small mark on the side of our goldfish and we were unsure if it had scratched itself on the rock in the tank or if it had caught something like rot. on the Tuesday morning we woke to find the comet had indeed died so we removed it from the tank, while doing so my partner treated the tank water for fin rot and fungi following the correct instructions. Within the hour all 3 remaining fish looked dead they were either floating or laid motionless on the floor of the tank. I wondered if the treatment was to blame don't ask me why it was just a gut feeling i had. I quickly got a clean bucket and filled it with cool water. in turn i fished the 3 fish out of the tank not one sempt to have any gill movement and none of them moved in the net. when i placed them in the bucket they either floated or sunk to the bottom of the bucket motionless. Amazingly over the next hour all 3 fish came round i was amazed as i thought they were all dead in fact i was 100% sure. Last night my partner totally cleaned out all the tank not jut partially but the whole tank and washed all the ornaments and glass. we have put all 3 remaining fish back in the tank and the seem fine they are swimming about and feeding. what do you think could have caused this with the fish? What happened to the fish when they weren't moving? have we done the right thing putting them back in a clean tank? do we need to treat the water for anything? and finally how do you see there outcome? do you think they will be ok? i would be grateful for any advice you can give me as i am totally confused by this whole situation. thankyou, lisa walters.

Answer
Hi Lisa, this could be a number of things but most probably your water quality levels in your tank got high and your fish get really irritated and can get very sick from this. You shouldn't have cleaned out your whole tank. You should always leave at least 10 of your water in your tank no matter what. But it should be fine now hopefully. I would bring a sample of your water to your pet store and they will tell your exactly which levels are off and tell you which supply to get to fix it. I would also purchase aquarium salt and place it in your tank. This makes your fish more immune to diseases and makes your tank healthier. Good Luck!