African Cichlid section!

Well it's here now, so let's see what we can do to help.
Hi mireme
I took a few minutes to look at your earlier posts and think you are having a problem that many other new fishkeepers have, that is, bad advice from the lfs (local fish store.) I suspect that your fish are dying because your tank hasn't been cycled completely, and that's going to be the first thing you must do before you can succeed with corys or any other fish. There's a little science involved, and I'm not the best one to help you with it, but let me give you a quick explanation so we will at least know that we're talking about the same thing.
For a tank to become "Established," it must have a colony of what we call Beneficial Bacteria growing in it. Most of them will be in the filter. When your fish eat and produce (harmful) ammonia in their waste products, some of the beneficial bacteria will consume it and produce NitrItes. These are also harmful to the fish, but other beneficial bacteria will grow and consume it. When they do, they will produce NitrAtes which, in small quantities, will not harm your fish. Excess nitrates will be removed when you do your weekly tank cleaning and water change. Ammonia and NitrItes (even just a little) will kill your fish.
There are two ways to cycle a tank. The first is called a Fishless Cycle and this is done (before adding fish) by adding ammonia to the water to get it started. The second is called Cycling with Fish, and this is likely to be what is happening in your tank right now. Unless you (or your lfs) do water tests and the results show up as 0 ammonia, 0 nitrItes, and some reading on NitrAtes, the tank is not "Established, aka Cycled." This is what the earlier posters were trying to find out.
Never trust an lfs that simply tells you your tank is OK without actually giving you the numbers.
I hope my explanation is of some use to you. The members who post in the New to the Hobby section can give you more specific help if you need it. If not, please let me know and we can try to help you with specific advice about corys in this section.