There is an easy way to add nitrates and phosphates etc. Most lfs will sell complete fertilisers, those by tropica and ada are superb. Or you can get the dry powders and mae your own.
Aim for 1 to 2 ppm PO4 and 15 ppm NO3 after dosing, I have found these levels to work well in discus and angel tanks.
This is a link to a UK based shop that sells the powders to give you an idea http://www.aquaessentials.co.uk/dry-...4_145_146.html and this is for the liquid ferts http://www.thegreenmachineonline.com...id-fertilisers
Good luck