Need Answers on what to do about plunging PH

Ok my 36g corner bow's water has been between 7.0 and 7.2 since I got it. I did a water change last Sunday and the ph registered 6.4 on Monday . The tetras don't mind the lower ph but I read that platies like the water more alkaline and the other fish I have in the tank like the ph to be atleast at 7.0. They all seem fine but should I raise the ph a little? I have a bottle of true ph which is suppose to help raise it. Can anyone give me some input this true ph stuff is suppose to be strong so I have to be careful using it I have live coral in the tank which has been keeping the ph stable am wondering why the ph plunged as it did.:help:

Ok the tank is a freshwater...I guess I misunderstood the LFS the coral I believe is a skeleton. I was told by them to add it to the tank as this would keep the water ph stable and it has been since I started. I started cycling the tank in March and it has been fine it cycled perfectly. The ph of the water has been 7.0-7.2. Tap water the same 7.0-7.2. The fish seem fine all my water parameters have been fine ammonia 0 nitrite 0 nitrate I believe was 20. The fish are fine no problems a reason why I am afraid to add this true ph to raise the ph. I only used it once when I first started and haven't since the ph was a little low when I first started. I do water changes every 4 weeks about 25% I change out. I use liquid tests and they haven't expired. I also have strips which are less reliable but wanted to make a comparison the strips tested about the same ph around 6.4. I am going to test the tap water again tonight but am inclined to do nothing since the fish seem fine. Thanks for the input much appreciated.