You don't need CCleaner, or any other app like that. OS X is not Windows, so all of that third party stuff you used to need are no longer needed (or wanted). About all you need are
Onyx (
www.titanium.free.fr) Free and does all the "cleaning" you need. Run it every few months or so (I run it twice a year unless I have a specific reason to do so). Get the right version for the OS on your iMac.
Ghostery (ghostery.com) Free-- blocks trackers, beacons, ads, malware and other stuff your browser may pick up. Turn it on to block EVERYTHING and then if you need something you can pause the blocking for that one thing.
Malwarebytes (malwarebytes.com) Free-deletes any malware that may get past Ghostery. Run it once after install, then again if you see any strange behaviors.
No need for antivirus, there are none in the wild at present that can run in OS X or macOS. You don't need memory managers, memory cleaner, hard drive cleaners, or any other of the crapola that routinely advertises for macs.