Hi GuppyDude
Breeding corys is fun, and not too difficult, and a 20 gallon tank is big enough to grow out a batch of them very nicely. If you will be using that tank for other fish at the moment, a 10 gallon will do well for them for their first few months too.
This is how I set my spawning tanks up:
First I set up the tank with gravel from the bottom of my main tank and take a bit of filter material to start a box filter running. This will provide a fairly safe environment for the parents to live in until they spawn. I like the gravel on the bottom because it acts as a deterrent to their laying eggs there and crushing them when they move around.
Do you have an air pump? You will need one to run the box filter, then an airstone, and later a sponge filter.
After they spawn, I move them back to the main tank and remove the gravel. At that point you will need to have an airstone to circulate the water over the eggs, This will keep them from getting fungus. They hatch in about 4 days.
Once they have hatched, you will not need the filter because you will be doing daily water changes to keep the tank clean. It will not be cycled at this point. An airstone just moving the surface of the water will give them lots of oxygen. (This is the stage where it is much easier to have them in a smaller tank.)
After a week or two, I attach a sponge filter and let it begin growing beneficial bacteria. Other harmless bacteria will grow on it too and the fry will eat them.
When they grow big enough not to get sucked into it, you can add a splitter to the airline and add a box filter too. This will provide much more mechanical filtration and, along with the sponge filter, will get the beneficial bacteria you will need started. At this time they are also big enough that you might want to add some sand or fine gravel to the bottom.
Another thing to think about before they actually spawn is what you will feed them. If you can get microworms, I strongly recommend that you get a culture started so that you have a good supply of nutritious live food for the fry.
I know that this is a very brief description of how to set things up, but I think it covers the basics. I'll be happy to answer any questions you may have too.
