Hi John44,
Unfortunately there are no great ways to “fix” a mattress that is too soft or has developed soft spots or impressions but some of the suggestions in post #4 here may be helpful … even if only on a partial or temporary basis.
I would also be very cautious about the Prana mattresses. You can read more about them in post #3 here and in post #2 here and in post #3 here. A forum search on Prana (you can just click this) will also bring up more information and feedback about them.
Phoenix