I highly doubt that adding decor to the tank would change the temp of it that much. If you are really looking that far into it I guess that at first the heat source would have to work harder due to the flow of warm air to the cold surface of the plants, heating them up. But once they are at the same temp as ambient I don't think it would have top work much harder at all. Maybe others have experienced a similar thing an I am wrong, this is all me thinking off the top of the head.