Shrimp

Cleaner shrimp set up a station and pick parasites off any fish that will hold still for it. Peppermints eat aiptasia, though not reliably and not every individual. Both are sensitive to salinity swings and to copper, so drip acclimate them slowly and never treat a tank with them in it.
Drip acclimate them, properly. Shrimp are the animal most often lost in the first hour, and almost always for the same reason: a fast transfer into water of a different salinity. A shrimp that goes in too quickly will look fine, then die two or three days later. Give them a slow drip over 40 minutes minimum and match salinity before anything else.
What each one is actually for. Cleaner shrimp set up a station and service fish, which is the closest thing to parasite control you can buy that is not a medication. Peppermints are bought for aiptasia and are genuinely hit and miss — some individuals never touch it, and a well-fed peppermint has no reason to try, so add two or three and keep them hungry. Fire and blood shrimp are display animals; they hide more and clean less.
No copper, ever. Not in the display, not as a residue in rock or sand from an old treatment. Copper kills invertebrates at doses fish do not notice, and a tank that was once treated can leach it for months.
They also moult, leaving a perfect empty shell that people mistake for a dead shrimp. Leave it — they eat it back for the calcium. If you are adding shrimp as pest control, read the coral pest guide first to confirm what you are actually dealing with.