ultrasonic cleaner solution

tired of trying to clean bolts and stuff with solvent so i bought a vevor 15L ultrasonic cleaner on sale for 169 shipped. 1 yr warr. looked on you tube for what solution to use but am more confused now. also see where some guys put the nuts/bolts in a glass jar/plastic container to save contaminating the solution. if you just use water and ???, what difference would it make for contamination? what do you add to your water to help cleaning if you don't use a container? i see everything from distilled water, water/vinegar mix, water /dawn soap. it is confusing to me. any help/ideas are greatly appreciated. tia.