ProtoSlo is the brand name of viscous, watery, non-toxic, methyl cellulose solution that's used to slow down protists (unicellular eukaryotic organisms) during light microscopy procedures. Most unicellular protists have motility mechanisms optimized for the normal viscosity of water, so they will be rendered quite motionless if placed on a more viscous medium.
Another brand name for this kind of methyl cellulose solution is Detain.
It is a viscus fluid that you squirt on microscopic protist critters to slow them down. It's especially important to squirt on the fast moving guys, like sperm and spirogiyra -- that way you can see them through the microscope without difficulty.