I have plastic 55 gallon barrels that I keep my water in. They are dark so there's no way for me to tell externally what the water level is. What I want to do is Mark the inside of the barrels with 5 gallon marks. My question is, what should I use to make said marks? I thought about a sharpie, small pieces of colored acrylic super glued to the sides, and even a paint marker. I'm worried that the sharpie and or paint Marker would leak chemicals into my water. The acrylic doesn't seem like a horrible idea but I want to know what y'all think.