How to make equal density pool dye without diffusion
Whole milk with food coloring solved my problem.  Whole milk has a specific gravity of 1.01, which allowed it to settle to the bottom around the main drains.  The diffusion rate was extremely slow over regular pool dye, which allowed the dyed milk to linger around the drains for an extended period of time (5+ minutes) to allow for the detection of the slow leak.