FEEDING

My plant is 8 weeks old and growing in a 20L pot,its still at the veg stage,how many litres of water should i feed it and how often should i feed it nutrients?Which is best,tap water or purified water?

Your not using the right kind of soil from the looks of it. You want a well draining soil w perlite. Water till slight runoff and don't water again until the pot is light.
Some soils like ffof have nutes already added and you can just use plain water for the first month.
If your soil does not have added nutes you should have been feeding from the start.
How much to feed depends on the stage of growth. For veg 400ppm or so, for early flower 600ppm and late flower around 800ppm. These are just close estimates. Individual plants will need more or less depending on lighting, genetics etc.

If your water is 300ppm or less it should be fine to use. More that that you might benefit from a ro system.