I'm not disputing its usefulness to plants, only that very high nitrate readings have been shown to harm fish. 50ppm is already seen as unacceptable by many fishkeepers. With those levels in the tap water its difficult to keep the nitrates under 100, in my last tank, the nitrates eventually went off the scale despite religious weekly water changes. I want to avoid that situation in future. The peace lilies were a very cheap and easy solution but I had no idea they would work quite so well as they have. I will be informing all goldy owners in my area, I know many of them use lots of expensive water treatments.
Honestly my main reason for not wanting to increase my water change an extra 50 litres is that the box I use to store the fresh water only holds 100 litres and I have a good system going. Would it be better to do twice weekly water changes instead?
Thanks for identifying the problem, now I know whats wrong I can do something about it. My plant whispering skills are clearly not up to it yet.
edit: oh the reflector is just the stock one that came with the tank, its spread doesn't seem to be very good. I did have a diatom problem when I first set up the tank but it passed. The tanks been clear for a couple of months, this only started happening about a week ago.