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Just put the drum onto the bathroom scales and it comes in at 18.2kg (incl. the container of course) so I guess some water shall be needed.

Should I bring it up to 20L or all the way to 23L??

I don't know what brand FWK it is. The HBS had a big pile of them in umarked plastic containers.

All the FWKs I have seen are 15L and ask you to add 5L of water (tip, use cooled boiled water, DO NOT add tap water!).

Thanks for the tip. Shall put the kettle on.