First, a three gallon tank is WAY too small for the fish you just mentioned. Just the fancy goldfish needs a 20 gallon tank, they get big and produce a lot of waste.

Second, did you "cycle" the tank? A new tank lacks the beneficial bacteria that, like the bacteria that helps you digest food in your stomach, digests fish waste (ammonia) into nutrients plants can use. When you set up a new tank, add just a few fish at a time a couple weeks apart, so the bacteria population can grow.

Lastly, Cory catfish are social creatures. Now, this is not what killed your cory in four days, but for long term Corydoras catfish health, they need to be in groups of at least three or four.

Here are my recommendations:

1) Keep the guppies in the 3 gallon tank. That is big enough for a single pair of guppies and a few of their babies. (It would actually be better if you have 3 guppies, one male and 2 females to spread his affections around!!) You can add a few cherry shrimp for added interest.

2) Buy a 20 gallon tank for the goldfish, after a few weeks (to cycle the tank) buy one of two more (small) ones. They will grow up together fine in a 20 gallon UNHEATED tank. (They like cool water and will not thrive in the heated guppy tank.) If you cannot do a 20 gallon tank, get rid of the goldfish. (But, DO NOT turn it loose into the wild as that could kill 1,000,000s of fish. You can give him back to the store, to a friend or as a last resort, place him in a small container in the freezer. I know it seems cruel, but is the most humane way to euthanize a fish and a billion times better than turning him loose, along with diseases he might carry, into a lake or stream.)

3) If you really want Corydoras catfish (And they are cute!), 3 or 4 of a smaller species will do fine in a 10 gallon tank, or will do great with guppies in a 20 gallon tank, you can even add a betta to either sized tank!

4) Last, DO NOT GUILT yourself over the cory. It happens to the best of us!! I’ve been keeping aquariums for 50 years and I, just a few weeks ago, did a major water change with fresh rainwater I gathered in the backyard. What could be better for fish than fresh rainwater? Well within a week 6 guppies and swordtails and one cichlid died!!!