As a keeper of fresh water tropical fish,I sometimes have to treat them.?

Itreat them on occasion with aquarium salt,wich I dissolve in water,then administor.My question is some of the partly dissolved solution was saved for further treatment,it was left for days.on returning to it,although some was still soluble in the bottom,the crystals had begun to crystalize and climb up the gl container almost covering the inner surface,Question; what is happening.