For the fish you are keeping 6.7 is fine, it is very close to neutral (7.0), your only off by .3 and therefore I would leave it alone if your fish are doing fine.
Guppies and other livebearers "prefer" water that is 7.0 and up, but if your fish are doing well then don't bother with messing with your water, because having a water fight will only cause stress for both you and your fish.
I have learned from many people that there is no such thing as a "perfect ph" that is foolproof and every fish will like it, that's simply not true. I used to think that at 7.0 every fish will like it because it's neutral, so I thought "hey, how can the fish not do well in 7.0, it's neutral, meaning it's not acidic nor alkaline!" Well, I have learned that is not true!
Also, from my personal experiences with water chemistry fights and confusion about it, I have learned that there are more natural approaches (THAT WORK!) towards either lowering or buffering the ph level of your tank.
My tap water is close to 8.0. I found out why. The odd thing is, my tap water is supposed to very soft, and the reason it is so hard is because the water treatment plant adds baking soda to the water to prevent old pipes from rusting. Within about 24 hours, the now de-chlorinated tap water (which of course is my tank), drops to about 6.0!
,I was always so confused as to why this is happening, and then figured that this is why my guppies, among others are do "OK", but not doing that well. I figured I could just use one of those chemicals from the lps to alter the ph (in this case buffer it).
So I was looking on different forums and websites, and of course using google and I ended up finding out about different buffering items that should work, such as: crushed coral, dolomite, oyster shell, and even baking soda.
So, I figured i'd go to the lps and see what they have, and sure enough I ended up finding crushed coral (which is very inexpensive). They had very fine crushed coral, but I figured "hey, it's still crushed coral, it should still work", and I bought 2 small bags of it. So I ended up putting some in my filter media bags to use in my filter, and then I also sprinkled a little bit onto my gravel.
Well, within that same day (maybe a few hours after adding it) I did a ph test and it read neutral!
, so then I thought alright, awesome!, so I did some more research on crushed coral, and found out that crushed coral will buffer your ph to a certain destination (usually 7.something, sometimes higher), and then keeps it there. (Stable ph, nothing wrong with that!), so I did a ph a test a few days after that and the ph went by .2, giving me a reading of 7.2 and guess what...??? it's still there!
, and I think that most community fish can do well in 7.2!
Wow, I never thought of a more natural approach until now!
I will never use those commercial ph altering chemicals ever again!
I keep my pH at 6.7, the lfs told me to day that 7 is best, but i've not had any issues with it and am not changing it now.
