Your fish would really appreciate a weekly 25 - 50% water change. It would refresh the tank and relieve some of the excess N assuming that you're not getting N out of the tap. Hopefully you will test for that as well as pH when you test your tap sample.
As long as you hold the status quo with the tank and don't need to dose large quantities of nutrients, a water change is an important option.
When extra nutrients are dosed regularly a large water change is no longer an option. Now it's a necessity, IMO. Certain elements, particularly traces can build up over time in a tank if it is not 're-set'. Over time, this build up can kill fish. You might want to consider buying a 'Python' water changing system. With this handy gadget you can change 50% of your 55 gal. tank in less than 20 mins. Makes it a snap.
Could be that the fish are raising the N levels. If that's the case or you find N in your tap water, you will need to look for a nutrient mix that has little or no N in it to balance what the fish or tap are supplying.
In a tank that is not injected or dosed carbon, the CO2 concentration is going to stay between approx. 1.5 - 4 ppm. If not injecting, the air stones will actually help draw CO2 out of the air into the water table. If/when you decide to inject, that would be the time to shut down the air stones.
The suggestion for the phosphate test kit at this time was to check your tap for P. If/when you decide to inject and up your dosing substantially, you would definitely need N and P kits. At this point you can take water samples to the LFS for testing if buying any of the kits is a problem.
Crushed coral can be put in a nylon 'filter bag', available at most LFSs. I would put 2 or 3 tablespoons of it in the bag and put the bag in the filter.
It is very slow acting and will not 'mess up' your pH. You probably won't see any change in kH for at least a week....possibly longer, but it will help raise kH over a much longer period of time than baking soda. Baking soda, btw, will raise kH over a very short period of time and dramatically, so be careful if you decide to use that. Add small quantities and test for content.
Len