I don’t think the BTA would appreciate much of a light change. How long have you had them on the sand now? Just asking cause if it just started… and you just transferred to this tank… it’s not unexpected. But I guess make sure your N and P don’t bottom out in the very least. Do you keep fish in there too yet? I’d consider those N and P levels and if you have anything that can fuel the so called “good” bacteria just enough to get more of a mixed population and balance to see some difference in the diatoms. I would just peruse that avenue for a little bit.
If it gets to be too too difficult, you could look into UV to knock the diatoms in the water at night (or day too I guess).
Apologies if I said anything you already know. Keep making music BB