So, I am using Eco Complete to create nice hard water with a high pH. When I do water changes, is it ok to add tapwater expecting it to even out with the existing water, or do I need to treat it to the correct pH before adding it?
It depends on how different the water parameters are from the tap to the tank. What are the parameters out of the tap & what are the parameters in the tank? KH, GH, PH & temp. In most cases you can just fill straight from the tap but if there is a big difference in PH or mineral content then it is best to treat the waer before adding it to the tank to avoid PH swings & softening the water for awhile. You fish wouldn't like that very much.
With a PH difference that big I would have a seperate tub/tank to use as a water preparation point. You would probably need to run a filter in the preparation point that contains either some of the Eco substrate or crushed coral in one of the filter compartments to bing the PH,KH,GH up to what's in the tank. Either that or carry out much smaller daily water changes so that the PH doesn't fluctuate too much.
Knowing the KH levels will be somewhat important here, for both the aquarium and your tap water.
The KH will buffer and restore the pH to where it is in the tank.
If your tap water has very little KH (eg <1 degree KH), then the aquarium buffers will win, and will restore the pH to the higher level.
If, however your tap water is high in KH, then it may be worth either looking at a filter that de-ionises your tap water, or using distilled water. Alternatively, you could boil the change water for a while first.
I agree with harpua and nutter, smaller, more frequent changes are probably easier to manage.