Mystery solved (I think)!
The Cichlids spent today getting more and more secretive (not sure if that's the word I want...), hiding amongst the plants and under a rock.
Did an
ammonia test this afternoon and it was pretty shocking - about 1.5
PPM!
I've done a 50%
water change and I can already see a change in their behaviour - they've started swimming around the tank and going for food now!
Obviously I'm going to have to do daily checks on the ammonia levels now, but how long will I need to keep this up for?