Thank's, really interesting!
I've cleaned the entire tank and everything in it, filled it up and emptied it again, then filled it completely. worked all night. Moved the tank to darker room to. I'll do water changes everyday just to be sure. Found à pic of the cave on a website to, and judging by that colour - mine has lost a lot. But i'll check with the store to, in case they have different colour set. Lookes like a regular plastic cave you can buy in any pet store.
They're back in the clean tank today, they all spent yesterday and night in seperate tubs. and seems a lot better now. The big one is even swimming a little, but the body still slitghtly bent to the left when he stops. But he's calm laying on his favourite spot most of the time.
So more research today! And à close eye on the little guys