What is the best app to clean up my iMac?

Hello, Pearlexx.

There are no good clean up apps for macOS.

There is no need to run any “cleaning”, “speed-up” or “optimizing” apps, nor any anti-virus apps. In fact, most anti-virus and cleaning apps are less than helpful on the Mac and often cause more problems and headache than they prevent because they conflict with the Mac’s built in securities.

Please see these Apple Support Community documents:

click> Effective Defenses Against Malware (John Galt)- Apple Community

click> How to install adware (John Galt)- Apple Community

click> Avoid phishing emails, fake 'virus' alerts…and other scams - Apple Support 

The best thing to do is keep your macOS up to date with automatic updates. These will only update the currently running macOS. It will NOT upgrade your OS to the next major version. It won't upgrade Catalina to Big Sur, for example.

Reboot your Mac at least weekly. This will release any RAM that has been bound by miscellaneous apps while the Mac is running.

Don't leave all of your browser windows open all the time. If you use Chrome, know that that browser is a memory hog.

If you have only 8-16GB RAM, don't run your Mac with ALL of your favorite apps open at the same time. Limit your use of RAM hungry apps like video and photo editors. Yes, it's convenient to have them open, but when your Mac starts to complain, then you've exceeded your limits.

If the dialog box says:

“Your system has is out of application memory”

and lists a bunch of apps to kill, none of which are using a lot of memory, then…

Then this is most likely being caused by background agents or daemons consuming excessive amounts of virtual memory.

Use Applications-> Utilities-> Activity Monitor-> View (menu)-> All Processes-> Memory (tab), and see what is using all your memory.

Looking at Activity Monitor WHEN you get the dialog is the best time to look.

Safe Boot, (holding Shift key down at startup), does the problem occur in Safe Mode? Could take 10 minutes.

Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, so if Safe Mode works try again in regular boot.

EtreCheck is a FREE simple little diagnostic tool to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac. It will not display any personal info.

https://www.etrecheck.com/

Pastebin is a good place to paste the whole report if you capture the URL while there…

https://pastebin.com/

Whew, they've changed pastebin & made it harder, but after pasting in, click Create new paste button, then Embed button, then copy the URL...

<script src="https://pastebin.com/embed_js/KuvnghqA"></script>

The important part of the above is between the quote marks...

https://pastebin.com/embed_js/KuvnghqA

Workable but harder for me to work with...the Note tool on the bottom of this editor's toolbar, as shown in the image, to copy and paste the output from EtreCheck. In a Reply before you click post, look for this to add longer texts...