sorry to get back so late, but I figured at or above 40mg/l since it was off the test kit charts and after I did a 90% water change it went down to about 5mg/l. I tested out of the tap when i did the water change and it read pretty much 0.
I have three tank and all get different food and whatnot, but all had off the wall phosphates. I can only assume that it came from the tap water one day, I hadn't changed anything in my routine for months and the water always checked out fine. I get the idea that the water company does whatever it wants as long as they are in comapliance with the EPA and FDA. Even then, our nitrates are consistently too high.
Somebody set us up the tank!
ALL YOUR FISH ARE BELONG TO US...