Seachem makes a couple of potassium supplements, Flourish Potassium and Aquavitro Carbonate. The Aquavitro product seems to be Seachem's answer to ADA Brighty K in that it adds potassium and increases KH to offset the buffering capacity of some substrates like AS. The least expensive option would be to pick up some potassium sulfate from somewhere like GLA or NilocG, a forum member who sells ferts in the for sale forum.
If you indeed have 20ppm of NO3 there's no need to add more NO3. Just be sure your test kit is accurate.
If you indeed have 20ppm of NO3 there's no need to add more NO3. Just be sure your test kit is accurate.