Bought a Malm and mattress from Ikea few years back. One of the worst decision ever. The mattress is horrible. The medium and hard mattress is soft. You want a medium hard you need to pay for the highest price mattress from them which is also so so only. Worst you can't find a mattress cover that fits (180x200) unless from them which is all plain and lousy low thread count cotton and not to mention expensive. The slat also deteriorated over time.

My advise is, if possible return it and get yourself a proper bed and mattress outside.