I have never figured out exactly how much nitrate my tanks are using per day, but some of the faster growing tanks will zero out the nitrate I add in maybe 4 days or so, so I would guess they are using just slightly less than 5 per day. I am guessing when my 75 gallon gets rolling, it will use more than that, since I have a lot of fast growers in there.
Two 1 GA desktops, a 5 gallon, a 2.5 gallon, a 10 gallon, a 20 gallon, a 25 gallon, and two 75 gallon tanks, all planted.
I am an expert on algae, so ask me if you have questions. I know how to grow it, just not how to get rid of it.