I answered this question on several other forums, thought you might find my reply helpful.
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As a watchmaker, I've used ultrasonic cleaners for 40 years or so.
I use plain, cheap Walmart paint thinner to clean guns and parts. (With plenty of ventilation). I also use lacquor thinner which cleans and brightens up parts.
For quick, flash cleaning of small parts, I put an inch or so of water, with some liquid soap as a wetting agent in the bottom of the cleaner. I then put small glass or plastic containers full of solvent into the tank. The water transfers the waves to the container. This is how watchmakers clean small items, without having to fill the entire tank. This also reduces the fire risk with inflamable solutions.
If you can afford it, the best cleaners are L&R brand clock and watch cleaning solutions and rinses. The cleaner removes light rust and tarnish, and leaves the item chemically clean. Instead of the rinse, (due to cost) use the paint thinner.
Be aware that some solvents will attack plastics, and ultrasonics speed that process up.
If you can live with water-based cleaners, mix most any good grease cutting soap with water and a little ammonia. Some of the "Orange" citrus cleaners are good.
After cleaning and rinsing, I use a hairdryer to dry, or a heat box with a light bulb inside as a dryer. Use of water-based cleaners can cause slight surface rust, so lube ASAP.
Always use the tank heater if it has one, use warm water, or let the unit run until the solution gets warm. A warm solution transfers the waves better. When possible, use a basket that keeps parts off the bottom of the tank. The tank works better this way.
KEEP YOUR HANDS OUT OF THE CLEANER. Too much dipping into ultrasonics can cause damage to your bones.