Just a thought, I am no expert. But, my tank is diagonally in a corner, but on the opposite wall I have some french doors and one day I noticed that the sun had crept round the curtain and was filtering direct sunlight on my tank.
I remember thinking, I don't know if this is ok or not .. but it looked natural and the fish didn't mind and didn't get up immediately to close the gap. But generally during the day the curtains are closed because I am at work and I was concerned about sun hitting the tank.
But a few days later my Danios went down with a case of Velvet. I have since read that the photosynthesis (sorry about spelling) aids the production of the Velvet organism ... could this be related to me seeing the sun on the tank that day???