I use one for brass with lemishine (?) and dish soap.  I used to use one to clean camera parts with some TRI 111 solvent. I have never done something like an action or steel parts.

For steel, use dawn dishwashing detergent and a bit of ammonia if you can stand the smell.

For brass, dawn dishwashing detergent and lemishine.

Don't need a lot, a teaspoon or so of detergent for a couple of gallons of water mine holds.

For small parts, you can put them in paint thinner or other solvent in a sealed glass jar, and then submerge that in the ultrasonic.  The sound waves transfer through the glass and work well on the contents.

Why not put the glass jar on the bottom of the tank?  The standard method for cleaning stuff in a laboratory is to put the pieces needing cleaning in a glass jar or beaker, put the cleaning solvent in the jar/beaker, and put the jar/beaker in the ultrasonic tank.  Only caveat is don’t let the tank run dry, or everything glass will break.

The commercial ultrasonic cleaners work good. For brass, I used the Lyman stuff because it is available at the store near me. I have also used citric acid and dish soap that worked OK. I just picked up some of the rcbs stuff and it seems to work the best so far.

For cleaning pistol barrels and actions, I use a 1:10 solution of Ballistol (dilution rates are on the can). It make an old 1911 look like it was new from the factory. My martini would fire on closing, and j couldn't fine instructions to take it completely apart, so I threw the complete fire control group in, and it cleared the gunk out completely. You want to get all the water out, but this leave a nice, thin coat of oil.