Although the difference in buoyancy is about 2.5% of your body mass, there is a 20-30% decrease in viscosity in fresh water. This means there is far less drag. What that converts to is that when you are 5 pounds negative in fresh water, you sink a hell of a lot faster than when you are 5 pounds negative in salt water. People then erroneously conclude that they were way 'more negative' in the fresh water, when in fact they were just as negative, but sinking faster. So, if you want to hit the same 'terminal' sinking velocity in fresh water, you will need to drop your weight way more than 2.5% of your body mass.
Think of the analogy with air. An inflatable human mannequin, weight two pounds, is two pounds negative in air. That same mannequin will fall much faster in air, than a diver who is two pounds negative in water. All because of drag & viscosity.
Likewise, each stroke of your fin or monofin will produce less thrust in fresh water. Again, think of the analogy of air. How much thrust does a fin thrust produce in air? Near zero.
Eric Fattah
BC, Canada