By adding Methane( a certain quantity) we can reduce it's density..
Many things have lower density than water. Most petrochemicals do, and wood does, as well. Anything that floats (not like a boat, though) has lower density.
You need to add more water so that the salt to water ratio is less Mehwesh T
At normal temperatures and pressures methane is a gas, water is a liquid.
nometals have more density because there is more elements put into the substance