[Timothy Terwilliker (OP)]

Is there a chemical that decreases the density of water?

[Kryptid]

Ammonia will do it: https://en.wikipedia.org/wiki/Ammonia_solution

As will ethanol: https://www.engineeringtoolbox.com/ethanol-water-mixture-density-d_2162.html

[RD]

Deuterium-depleted water is less dense than ordinary water.