differences in ph

Question
i have tested the pH in 3 ways and all results are different?
my 125 litre community tank has 3 large pieces of bogwood and live plants so i do not want to use chemical buffers. my fish are 4 gouramis, 16 neons, 4 Corydoras cats, 6 Mollies and 2 clown loaches.
direct from my taps i get 6.6
direct from the tank i get 6.0 - 6.4
test from bucket of tap water left overnight is 7.6

i change the water 20fortnightly and add tap water conditioner.
will the pH cause a problem being so low?
why is the overnight tap water have such a high pH
might this also be the reason i cannot keep guppys alive?

Answer
Dear Craig,
This is quite interesting.
I would highly suspect the reason your aquarium's pH is low than your tap water is due to natural organic matter decaying and producing acid in the aquarium and especially the tannins leached from the Bogwood. There really is no worry to it as long as your gouramis, mollies, neons, corydoras, and loaches are healthy and vibrant. But if course the Ph should never be allowed to drop too low and this can easily be prevented with frequent water changes. It might be a little better if you did water changes twice a week instead of fortnightly or even once a week would be much better.

The low ph could certainly be a reason why you cannot keep guppies alive. They thrive best in a pH of about 7.8 or higher but they can adapt to lower pH levels, they just do better in higher pH. It could be that your local fishstore's guppy tank water could be vastly different in terms of chemistry to your aquarium's this is something you might want to check into.

The ph starting out reading low but then raising higher overnight could be due to carbon dioxide degassing.

Besides that, sounds like you are doing everything right! I really hope this helps answer your questions!

Best wishes and Happy fishkeeping!
Karen~