Thanks
@Hippie and
@badbee for your remarks. I don't think the products you mentioned are available where I live. But good that you mention it, so others can learn about it. The thing is, this Limpuro product is explicitly for cleaning bongs. That's the purpose. I thought the whole idea behind this product is to be good for getting out all the nasty stuff without any additional solvents, without any mechanical cleaning, without any fuss. Indeed the manual states: "For heavily soiled hookahs, use a limescale remover such as citric acid for thorough cleaning." I overlooked this info because my bongs aren't heavily soiled. I clean them every day.
Anyhow, I'm willing to learn. So over night I've tried
@badbee 's advice to put some iso into the mix. The result was not better than with pure iso. I started with iso, than mixed it with hot water and the Limpuro. I think the iso just gets too diluted in the mix and I don't want to put more in it than I would with a normal iso-only cleaning (because what's the point then?). Also, with iso-only I can reuse the iso, which seems difficult with the Limpuro-iso-mix.
I will keep trying a combination with limescale removers like citric acid next week. I'll give an update.