Time of day does make a difference especially on PH. Also if sample was a true sample of the entire water column or a top or bottom sample. A top 12 inch sample for nitrates in warm water on sunny days will test lower due to planktonic and FA uptake or bottom water may be higher with decomposition and or decayiing OM. Nitrates less than .05 ppm help keep growth to a minimum.I prefer Ortho P levels less than .01 ppm but get samples that can be as high as .34 Kevin Ripp from AquaFix has a great article on nutrient management in the Nov/Dec 2012 Pond Boss Mag starting on page 22. Check it out
