Firming up the mattress

OK, we love our queen size bed in our 25' CCD. I've read many threads about people who have altered the mattress by adding foam or memory foam. They indicate that their actions make the mattress more comfortable. I'm not quite sure weather this increases the firmness. I would like to increase the firmness of my mattress and avoid the cost of a new mattress and still utilize my existing fitted sheets and liner. All Advice would be greatly appreciated,



Thanks,



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