What is the best sump filter media? I am thinking of building a sump filter out of my 65L tank to filter my 400L tank with two Os and a plek. Would a powerhead from a canister filter work? It 1200L/ph

You need several kinds of media. Mechanical, biological, and chemical(optional).

Mechanical- Usually some sort of filter pad or sponge to block out particles.

Biological - Bio(Bac) balls and rock to offer surface area for your nitrifying bacteria to grow on.

Chemical - Usually carbon to filter out the toxins in the water. Many people don't use any chemical filtration (myself included) and never have problems.

i wouldn't bother with a sump for tanks less than 200G(uk) it just isnt worth it.

Marcus is right. Sumps are best used for big tanks, and a quarter of the capacity of the tank should be used as size for the sump.
If you do go through with it, you can use loads of materials.
I have mine filled with lava stones, other porous stones, some filterpads,mini scrubbies and ceramic rings.


My pre filter is filled with filterfloss, a foam pad, topped of with white cotton wool.
But a big canister may be the best option for your tank.