Why the increase in pH?

Brief：
I am setting up a 4-ft tank with ehiem professional 2 canister filter (model 2028). I intend to rear a arowana. The ph of tap water at my location is .. 

I am setting up a 4-ft tank with ehiem professional 2 canister filter (model 2028). I intend to rear a arowana. The ph of tap water at my location is 7.7. After the set-up, the pH has shot up to 8.5! What is the cause of this? How can I effectively lower the pH to about 6.5?

Arrowana need a ph of 7 or lower to be comfortable, Have you checked for ammonia as this can increase the ph by making the water alkaline. Try to introduce nitrifying bacteria from another tank or filter to consume any ammonia. Nitrites and nitrates are produced by these bacteria hich are far less harmfull than ammonia and they also feed any plants you have in your aquarium. This is called the nitrate cycle and it is important to get it balanced before you put any fish in. Once the Ammonia has gone the ph should come down to what it was before. Try adding peat in a bag to the filter or putting Bog wood in the tank to increase the acidity.

You will need a very big tank as arrowana can grow to over three feet in length and can jump over the length of their body out of the water. In the wildthey do this to catch insects.

A care sheet on setting up an aquarium is available on my web site. Have you added any rocks recently? Many times minerals in rocks will raise the ph. For an arowana even 7.7 is a little high. Consider putting peat in your filter to lower the ph some. Or you could use chemicals to lower the ph but that becomes expensive. Testing the pH of your tap water right out the tap won't give you an acturate measurement. Collect the tap water and test it after 24hrs (allowing the CO2 to escape), and you will get a more accurate pH reading.

Peat or media containing peat will lower pH.