I personally wouldnt use food coloring in my frog tank. A False Bottom style tank doesnt require drainage material to be down in the water area. The LDL stuff you are using from JF is meant more for a Drainage Layer setup.
If you want to keep the LDL in your FB you could add a little bit of Blackwater extract to the water so it tints it a light brown. Than should help you see the water line easier.
Also - Stick your finger into your substrate and move it around some. If a smell of rotten eggs comes out the soil is bad and needs to be replaced. When soil is allowed to get water logged, like what happened to your soil, it quickly starts to grow nasty bacteria and should be replaced with fresh soil.