If your pH is already 6.8 then I don't know how much lower adding CO2
would take it. Certainly the effect will not be nearly as drastic as if your tank was pH 7.6 or something like that.
The short answer as to what to do about it is, increase the buffering capacity (KH
) of your water. I would avoid adding chemical to do this like the plague; far too easy to screw up and have wild pH swings.
A much safer, more stable, method would be to add crushed coral to your filter, if that is at all possible. Easy to do with canister filters, and easy with quality HoB
filters like AquaClears. If you use a HoB
filter that uses all-in-one inserts, or if you use a biowheel, sponge, internal, or undergravel filter, you might be in trouble. In that case, probably all you could do would be to put a bag of crushed coral in the tank itself somewhere.