I have a 400litre tropical tank with malawi cichlids. The ph in the water keeps dropping to 6. The last time this happened I brought the ph back up to 8 using ph8.2 powder. I am due to change the water tomorrow but checked the water again out of curiousity but find it has dropped back down to 6 with no water changed yet. I have coral sand and the proper rocks for a malawi cichlid set up. I even bring the water I change with up to ph 8 before I add it.

I have another smaller tank in the house and it stays at a constant ph 7 all the time.

Can anyone help?

Are you positive that the coral sand is real coral reef?
This doesn't sound like typical behavior for an environment with coral substrate.
Your ph should be staying at 8.2.

I would go back to the place you bought the sand from and check to see if it's real coral reef.
If it says coral safe or imitation coral, it is not real.
If it says coral mix then it does not contain enough coral to sustain the ph for your size aquarium.

If you need to replace it, I'd go with 100% crushed coral reef instead of coral sand, I've always had great success with it.