Question about Tefal Smart'n Light

I don't know if there is an additional one that you can buy. But your assumption that large chunks of limescale will seep through if you don't descale frequently is correct. I still can't do that with my pre-hardened kettle and am therefore a bit disappointed. It's probably a matter of getting used to it. If I haven't descaled, I usually use a normal sieve as well.