Planted Tank Enthusiast
Join Date: Feb 2003
Location: Minneapolis, MN
I've heard that you can, though I've never used a Magnum filter myself. You don't need the diatom powder, but it makes everything go faster and will extend the life of your micron filter (because you can rinse it off and all the gunk is trapped in the powder). The diatom powder alledgedly will let you filter out smaller particles and even some parasites like ick.
The cheapest way to get diatom powder is from someone who has a swimming pool with a DE filter. They use gallons for each reacharge of there filter, so are usually happy to let you take a small amount.