Thanks Joo and Orient for your reply. I got myself some small corals and wash them for some time. Added a small quantity into the 2nd last layer in my filter. Check the ph yesterday and today. Register at 6.5 ph. I guess this is ok for discus. Without this my water ph will drop very fast. Even down to 5 ph. I am still keeping an eye and testing the water ph and kh closely. I also added some coral to my plant tank since it can get quite acidic from the DIY co2. Now, I wonder what do I do with the rest of the corals.

Frankie