I noticed that when comparing the colour of the water against the API colour chart, there is a variation of colour depending on whether the tube is held right up against the chart or slightly away from the chart. The colour is lighter when the tube is a about an inch away from the chart due to light behind the tube. This makes a particular big difference to the nitrate test; it appears to vary between 10 and 40 as you move the tube away from the chart. So can someone please advise on the best way to take a reading.