I agree with IMCL85. ALthough a pH of 7.8 is not optimum for tetras, they will adjust fine. They will do much better with it constant there than with the fluctuations you would get with trying to lower it. If you want to bring it down a little in a very natural way, add a piece of driftwood. Looks good in the tank and will also lower your pH slightly but still keep it consistent. Learning what your KH is (as tttnjfttt suggested) is also a good idea. That will give you a good idea of how stable our pH is incase you make any changes later. The higher the KH, the more buffering capacity you have and the less effect changes will have on your pH.