The content of the computer's hard disk drive, includes storage of your created works
files you made, images or music files, and the likes of which some flakey application
such as 'cleanmymac' won't really help with. To clear the problem, you'd have to invest
in an externally enclosed hard disk drive, and move/copy those items you sought to keep
to the external drive, then trash the originals from the computer's hard disk drive.
Or upgrade the internal hard disk drive to a larger capacity, and have an expert move
the content from the old drive into the new one, if you aren't skilled to do that task.
Also, the lack of free or unused hard drive capacity can be a problem with systemwide
performance; that is because the hard drive free space is usually available to the OS X
for use as Virtual Memory on an automatic level, in cases when there is not enough
actual chip RAM installed. The slower read-write to/from the hard disk drive takes the
place of suitable memory chip capacity, at a poorer rate of return compared to real RAM.
If your computer has less than 15% unused hard drive capacity, it will have issues; this
may vary depending on what you are using the computer for, & your open applications.
Back to RAM memory chips, if your computer has less than 6GB RAM installed, it may
be able to see a slight measure of performance gain (after removing excess files to
an outside hard drive) by upgrading this by use of suitable quality RAM chips. The OS X
versions after Snow Leopard 10.6.8 prefer more than 4GB and up, to work adequately.
•iMac: How to remove or install memory - Apple Support
Maximum Memory 6.0 GB (Actual) 4.0 GB (Apple)
Memory Slots 2 - 200-pin PC2-6400 (800MHz) DDR2 SO-DIMM
{the link above suggests less than factual upgrade capacity, so to
expand that to more than 4GB, you could use one 2-GB chip and
one 4-GB chip for a total of 6GB; or choose two 4-GB and not be
able to really see a gain from the rounding up to 8GB in your Mac}
Most credible Mac user advice, says to not use Mackeeper, Cleanmymac, or those
other non-productive third-party products; they don't really help the overall issues.
And they are very hard to completely remove; their remaining bits do affect OS X
performance and do more harm than good. So there are a few advice sections
in these ASC discussions where removing those kinds of badwares are mentioned.
Information can be found about adware, malware, and performance issues on the
pages of http://thesafemac.com and note the Tech Guides; plus link to adware medic.
You may be due to have the hard disk drive in the iMac replaced, if it's an original part.
They do wear out and when that happens, your data can all be lost. So address that
by the best means available. An iFixit.com repair guide for the build model year iMac
you have is online and can be used for free. You'd need to see it, though, when or if
the computer in use is taken offline for the procedure. Or let an expert do the upgrades.
In any event...
Good luck & happy computing! 🙂