Hi Sudhirr, does your tap water contain nitrate? If not, you can fill the tube with 2.5ml tap water and the other 2.5ml tank water which will dilute the tank's nitrate by 50%. It should shift to a different color on the chart that's hopefully easier to see then just multiply your result by 2 for actual nitrate level in your tank.
edit: your pic looks like it's in the 20-30ppm range but they don't always come out the same on a pic. Also the tube is in the shadow which makes it darker.
For me, one way I tell the difference between 30 and 40 is looking at the shade of red. 30 still has hints of orange, while 40 is pure red.
Last edited by Betta Splendid; 08-25-2020 at 04:05 AM.
Reason: additional info