Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I use Safe-T-Sorb.
This is the same material as the clay type of cat litter, but is baked a bit longer. This makes it harder, does not fall apart in the water, and it is darker in color. Much more natural looking than the white or off-white kitty litter.
Safe-T-Sorb is a product made to absorb oil spills, so you might find an equivalent product in stores that supply car repair shops. The manufacturer is Molton. I do not know if this brand and product is available in Norway. Something similar probably is.
If you want to try kitty litter get samples of all the ones you can and test them by placing them in a bucket of water and keeping the water moving with a bubbler or power head, or even just stirring by hand several times per day. After a week or two see how well they have held up. If the water is staying cloudy all the time, then the kitty litter is not a good choice.
Talk to other aquarium keepers that have access to the same materials as you, and see if they suggest any particular brand.