Maybe not all that much light at this time of year, (Pia's in England, don't forget) so much as just the plant slowing down and not absorbing nutrients as quickly due to the short, cool days?
Are you adding any fertilizer to the aquarium water? It does have some, with the fish waste of course, but maybe not quite enough. You might give a pinch or two into your aquarium water to boost the nitrogen a little.
I sure do agree about adding some magnesium, though. It never does any harm and orchids like it a lot.
