You have a few options:
Global makes a good ultrasonic cleaner, but it's fairly expensive at around $60 per gallon.
Blue gold works extremely well as a cleaner and degreaser, but it's expensive if you get it through a dive shop - on the order of $90 per gallon. However if you buy it from an aviation supply company you can get a 5 gallon bucket full for $125. It's used for cleaning jet turbine engine parts. Diluted 20 to 1, it will provide you 100 gallons of ultrasound solution for a buck twenty five a gallon.
Simple Green is a great ultrasonic cleaner and degreaser as well. It's main advantage is that is readily available pretty much everywhere and it's not expensive at about $15 per gallon. Diluted 16 to 1, that gives 16 gallons of solution at about $0.95 per gallon.
Simple Green Crystal is even better, particularly, for tank cleaning, as it's easier to rinse and doesn't have the strong smell. It only costs a couple dollars more per gallon. But it's harder to find. If you have Amazon prime however, you can get it for $17 per gallon delivered to your door with free 2 day shipping.
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All of the above are alkaline cleaners. Most dirt and soil types are acidic, so alkaline cleaners are very effective for removing dirt and grime. Alkaline cleaners are also the go to cleaning solutions for removal of oils, fats, greases, and proteins and consequently they are the default cleaners for O2 cleaning tanks and regulator parts for oxygen service.
In contrast, acidic cleaners are commonly used to brighten or etch metals and are effective at removing any mineral deposits or oxidation on surfaces. Acidic cleaners are also ideal for removing any starches, carbonates, and insoluble hydroxides.
Which is to say, you may need both to effectively clean a regulator with some oxidation on it.
A solution of white vinegar and water is an acidic cleaner (acetic acid) and it works great to remove heavy salt deposits that otherwise would pretty much never come off in an alkaline solution, without a good deal of wire brushing.
I'm neutral on 50% versus 20% vinegar and water mixes, as a 20% solution for 10 minutes will be as hard on a chrome plated part as a 50 percent solution for 4 minutes. But frankly even 50% acetic acid is not all that effective for removing chrome, so I wouldn't worry too much about it for brief soaks.
That's pretty much how I use it. I'll give the parts a brief ultrasonic cleaning in an alkaline solution to remove any oils, and then give them 3-5 minutes in a vinegar and water solution. That will remove any salt, mineral deposits and oxidation in short order.
Acetic acid probably gets it's bad reputation for doing exactly that - it removes the mineral deposits and oxides that have already attacked and compromised the underlying chrome and nickel plating, so it's common to expose the underlying copper plating on the brass. The acidic solution gets blamed, but the chrome and nickel plating was already toast due to the oxidation that had already occurred.
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You can use both alkaline and acetic acid in the same ultrasonic cleaner at the same time. Just put the vinegar and water solution in a plastic tub (or a large plastic drinking glass from Burger King, etc) and set the tub in the basket of your ultrasonic cleaner containing the alkaline cleaning solution (just make sure the level is low enough so the addition of the tub doesn't run it over). The plastic tub will reduce the effectiveness of the ultrasound slightly, but it's mostly the acetic acid doing the work anyway.
Mild solutions of acetic acid won't attack a stainless steel tank in an ultrasonic cleaner anytime soon, but it's not a good idea to leave the solution there long term.
