That fish hardly grow at an LFS is not that surprising, but that they don't in a 400 cms tank is!
Thing is that they are supposed to grow to 30 cms (Clog). FishBase even lists 50 cms TL.....
So when you're saying that yours are still around 10 cms like when you/we bought them, that could imply that they stay much smaller than indicated.
I for sure wouldn't mind
For some reason they remind me of Synos: they are rather elegant and thoughtful swimmers - unlike most Doradids I've kept in the past.
Btw: it's non of my business Bas, but they are not on your "My Cats" list.