We need more data to answer your question.
What is the pH of your tap water (assuming that is the source water)?
What is the pH of the tank water (which may be different)?
What pH do you want in the tank, related to what fish species are you intending?
It would also help to know the GH (general hardness) and KH (Alkalinity) of your tap water, as these are related to pH and we can better advise on the pH if we know what to expect because the pH in an aquarium will react subject to the hardness. You can find this out from your water supply people; no need at this stage to get a hardness test kit, yoyu may not need it again.