Hi norbie
I'm sorry to learn that your cory died.
When a cory becomes ill or dies, the most likely cause is bacterial infection. There are bacteria in your tank, at all times, that could infect your corys or other fish. The best way to prevent this from happening is to keep your tank clean and fresh and to keep your fish well fed and healthy.
This is like people catching colds or the flu. Germs can be all around us, but we usually stay healthy unless we get run down or have our immune systems compromised in some way.
Here are things that stress fish and make them more likely to catch bacterial infections:
Temperature fluctuations
High nitrites
Overcrowding
Overfeeding (so that there is leftover food in the tank)
Low oxygen content in the water
Infrequent or inadequate water changes
Aggressive tank mates
Sometimes bacterial infections can move slowly, but other times they can kill a fish before you even know they are sick.
It's hard to say just what caused your cory to become ill, and it could well have been the fact that your filter was off, but if any of those conditions exist in your tank, it would be a good idea to correct them for the good of your other fish.
Corys are schooling fish and do better the more of them there are, but keeping them healthy is even more important. At least the two remaining have each other.
For now, I wouldn't suggest that you add any more fish to your tank. It's not very large and the rapidly growing guppies will soon have it overcrowded. I would also suggest doing a couple of good water changes and bottom cleanings over the next few days to reduce the amount of bad bacteria living there. It's possible that the filter being off caused them to reproduce more rapidly than they would normally have.
