If you use KCl the plants will use the potassium but not that much chlorine, so it will build up in the water. For that reason I would much rather use KNO3, giving the plants both nitrogen and potassium. Or, you could use K2SO4, since plants do use both potassium and sulfur. Some people get quite a bit of nitrates from their tap water - they usually use K2SO4. Those of us who don't get nitrates from tap water usually use KNO3.