ph in my reservoir is always acid.
 

The ph in my reservoir (25 gal) is always acid. I'm using ph up like crazy. Changing the solution didn't help. After a few days it was acid again. I started using crushed coral. It's only been 1 day and the ph seems to be stabilized. The ppm is ok for now. What terrible things might happen?

If your pH is to high or to low your plants get lock out and can't utilize the nutrients in your solution. Thus your plants do poorly.