If you are in doubt about which mattress to choose to sleep on, you can find the average mattress hardness vs. softness in the last photo on each of our futon mattress product pages here. In addition, you can learn more about futon mattresses here.
If you want to add another mattress to your Karup Design bed frame, that is also possible. Our available bed sizes fit most mattress sizes that fit into the bed frame. However, we do not take responsibility for the fit of the bed if you choose to use a different type of mattress than our futons.