PH

Hello steven welcome to the forum

have you tested your tap water first to find out what ph it starts of from the tap leave a cup of water of night and test it the following day , this will give you a base figure for the ph of your tank , after that you need to find out the hardness of your water this should be listed on your water suppliers website if your water is soft or very soft like mine it is likely that your kh is also very low - kh helps to buffer the ph against changes like you are suffering from ,if you dont want to buy a test kit for ph you could take a sample of water to your lfs fish shop and ask them to test for kh for you.
i to was suffering the exact same problem as with my ph droping to  6.0 or lower overnight but there is ways to correct it once we know the cause 

Thanks I'll check tap water today and tomorrow after standing. I'll let you know the results. Yes I was concerned about sudden fluctations too.

Curious.

The reason they said food can lower the pH is that uneaten food decomposes to make ammonia which the filter turns into nitrate, which acidic. Lots of nitrate coupled with insufficient water changes can lower the pH.
Bogwood can also lower pH because it releases acidic tannins. But there would need to be a huge amount of it to lower pH that much.

I see you are using a test strip. These are known to be inaccurate. If you really do have a KH of between 6 and 10, you shouldn't be suffering pH drops like this - if it really is dropping. I have KH 3 and mine only drops if I don't do weekly water changes.
I am beginning to suspect the accuracy of the strips as nothing else adds up. I would be inclined to ask a shop to test your pH for you, especially if they use liquid reagent testers. Take a sample of fresh tapwater, stood tapwater and tank water.

Thinking about it, you don't really need fresh tapwater, just stood and tank. They might object to 3 samples but 2 should be OK especially if you say you want to compare your tank to the tap.