Leave the pH alone. Seriously. You will make your fish sick monkeying around with it.
You don't have to let your tap water sit in a bucket, that is what water conditioner is for. Simply match the temp of the tap water with the temp of the tank water, add enough conditioner to your tank to neutralize the amount of water you are adding, and put the water into the tank. Simple. I think the store just wants to sell you ro/di water. If there's nothing specifically wrong with your tap water, your goldfish will be better off with it; the pH will likely be higher and more to their liking.
Why don't you test your tap water's pH, both straight out of the tap and after sitting out overnight.
They won't be high maintenance once you learn what to do.