Logistic nightmare

[OrionN]

My 30 g tank evaporate .5 g per day. I have to add tap water as top off for two months before I get the dissolved solid I put in my tank with the initial make up of salt water using tap. What I am trying to say here is that the concentration of toxic dissolved solid in tap water (copper and other metals) are very low. It is the adding of these daily by top off water that is a danger not the low concentration initially when we mix the water with tap water. Even copper is a minor nutrient need for our reef tank, but very toxic to invertebrates only when exceeded certain level.
BTW, my tank is very efficient in handling nutrients and the nitrated in the tap water is of no concern to me. Just the dissolved metal, namely copper is of real concern to me.