You will still need to test - this product won't eliminate the need to test your water. In fact, forgoing water changes due to using a product like this would INCREASE your need to test IMO. I wouldn't take the companies word that the product works as well as they say it does
You will also still need to test for Calcium and Alkalinity, and if you aren't changing water at regular intervals you will need to start dosing these two things at a minimum (and again we arent even talking about trace elements here). Getting a two part solution mixed up, and dosed correctly is not a 5 minute project, and to me is acutally more tedius than a water change. I can change out 20% of the water on my 180 in about 20 minutes - changing water really isn't that big of a deal
As far as the tap water thing, stop using it and you'll solve a lot of your problems. Get yourself an RO/DI system and call it a day.
There are more effective, proven ways to deal with phosphates and nitrates in an aquarium IMO, including feeding less, macro algae, and GFO/Bio Pellets - which have an established performance over many years