I did a google translate on your questions. Can you post a picture of your tank??
If the pH in your tank is 8.0 then you have something in the tank that is raising the pH, (ei. gravel, sand, decorative stone or maybe filter medium) which should be removed/replaced.
Corys live in a wide range of pH, depending on species but if you can keep it at 6.0 - 6.5 that will work for most species.
I'm not sure if the large decorative stones are raising the pH of your tank, but you may want to tank them out and test them.
To check the stones, put them in a bucket of water with a low pH (4-5) and a little aeration and leave them for a couple days. Then check the pH of the water again to see if has changed. If the pH goes up then it could be that they are raising the pH for the whole tank, and you may want to replace them with another type.