Incidentally, what you should expect
to see is that nearly 100% of the physical memory is in-use for something,
all the time. You might, for instance, see many megabytes being used for "buffers."
Like most OSes, Linux is lazy.
Having brought something into memory once, it likes to keep it there until forced to get rid of it and bring something else in. Therefore, the amount of physical RAM that's being used at any particular time is not
an indication of "goodness" or "badness!"
What you would look for, if you suspected a memory-related issue, is any sign of pressure
being placed on the virtual memory system ... persistent swapping activity, actual memory-related waits being imposed on processes that otherwise would like to be running. If you don't actually see signs of pain,
so to speak, then there isn't a memory-related problem.