Disclaimer: I am in no way, shape, or form suggesting that people use tap water. Mine is higher quality than most I've seen and as such shouldn't be viewed as me condoning the practice regarding usage of tap water in reef tanks.
Now, the reason for this post:
My reefing experience is coming up on 2 years now and through that time I am seeing the exact same trend between my 20g that I started with and the 90g that I have now.
In both of these tanks I have been unable to feed my fish enough to impact my water parameters. No matter how much I feed, the readings stay at 0ppm of both nitrate and phosphate. In my 20g I was feeding 1/3 of a cube of frozen mysis daily to 3 fish. I moved that up to half a cube daily in my 90g with the same results. Due to a fish that's highly aggressive if he doesn't get frequently, I have increased my feedings to a large number of pellets twice daily with every other day nori. The parameters stay the same at 0.0ppm of both phosphate and nitrate. In both of these tanks the growth of my macro algae is extreme which is obviously what is consuming my nitrate and phosphate overload.
My point in all this:
Due to my using tap water which has higher levels of nutrients, most notably potassium than the ro
water that is used in most reef tanks it isn't limiting the growth of my macro algae.
Leibig's Law (Liebig's law of the minimum - Wikipedia, the free encyclopedia
would point to agree with me where it is summarized as "Only by increasing the amount of the limiting nutrient (the one most scarce in relation to "need") was the growth of a plant or crop improved."
Why does it matter?
A further method of nutrient control in our tanks could be as simple as a small addition of potassium.