[crazie.eddie]

You can add peat or wood, which releases tannins. But this makes your water light brown.

You can also use pH lowering chemicals, but I normally don't like doing it, becuase it may cause your pH to shift, during water changes.

What type of substrate are you using? Does it contain coral or seashells?

Do you have any seashells as decor?

Coral or seashells in an aquarium can increase the pH in the water. Have you tested the pH in your tap?

 

[crazie.eddie]

You said you have coral. Real coral will increase the pH in your water. Unless you want high pH, I would remove it. Cichlids and salt water fish are the only fish that require high pH.

 

[crazie.eddie]

Doesn't look like coral, but I'm not sure. But it could contain some kind of calcium carbonate. An easy way to tell is if adjust your pH is to test your water from your faucet tap. If it's lower than your tank, then something in your tank is causing high pH, which could be the rock. So then, basically remove the rock, replace all of the water and check your pH again.