What a tasty recipe!
I didn't have any fish stock cube but used 350ml of fish stock, resulting into a moist tagine: perfect served on top of some couscous.
I used some kalamata olives instead of black ones as I prefer their flavour.
You could probably use any kind of firm fleshed fish (had some basa fillets).
Oh, and I skipped the marinating phase as I was in a hurry. It was still a lovely dish! Got the thumb up from my blokes: DH, and DS (10 and 12).