My water parameters are almost identical to that and I dropped my ph to a 6.6 or 6.7 and that set my drop checker in the green. I suggest you pick up a drop checker also, that is the only sure fire way you will know if you are over or under dosing the co2. You can pick up one on ebay fairly cheap if that's available to you in Greece. Until then I would shoot a bit high for about 6.8 until you are sure. Mainly keep an eye on the fish for signs of a lack of oxygen. They will go to the top of the tank and gasp for air if you are over doing it.
EDIT: I hate when that happens, I replied at the same time someone else did.
Last edited by Vonzorfox; 05-04-2008 at 12:08 AM.