Suggest Remedy For Itchy Rash

I have had a itchy rash that doesn't go away . it fades a little if i dont scratch it. It seems more itchy at night. I have used tar soap Psorisis medicine but it doesnt go away. Now I just in the past couple of weeks got a rash in the crack of my buttocks. I was thinking I need to get a water softener but i dont know. Please help