5 gallons of 12% with a CYA of 50-60 in an 18K gallon pool would add about 33ppm of chlorine. Too much. Only add 3 of the gallons. That should add about 20ppm which is high enough. Test about an hour after you add it this evening and then test again within two hours of sunrise in the morning. What you need to find out is if you are losing more than 1ppm overnight. That information is an important piece of the puzzle.