As you flush little goldie down the toilet to his watery grave,you may want to consider what has caused your fish to go to the big u-bend in the sky.
You should always try to identify any casualties as early as possible. Every time you feed your fish, do a quick head count to make sure all are present and correct. Of course getting fish to stand still while you do a head count is trickier than it looks.
If anyone is missing, check they are not near the filter. A fish going into isolation like this is a bad sign. If your fish is presumed to be dead but you can't find it, check filter inlets or underneath decor.
If your fish is found dead you must move it as soon as possible otherwise the body will rot and release pollutants and if it died from disease you don't want the other fish eating it.
If you can move the corpse quickly it's worth giving it a once-over to try and determine the cause of death and try to eliminate the chances of the same happening to your other fish. For example maybe it shows signs of having been bullied by other fish. You must also test the water to see if something like ammonia has caused the death. If there is an imbalance you need to address it and possibly perform a water change.
If there is no issue with the water quality, then you need to explore other possibilities like starvation or stress. Of course death by natural causes shouldn't be ruled out.
Before you restock you should make sure you have brought water conditions back to normal. You should also wait a period of seven days to monitor what happens to your other fish.
