I have 3 LED light units set to 40% power, I can easily raise that but I'm trying to avoid problems with algae by using lower light levels. (The lights are in pristine condition and very high quality.)
Do you think it's because of low light levels? In my experience the higher light the more diatoms I get.
After increasing light, the algae may briefly increase, as it responds faster than the plants. Takes about two weeks for a new balance to visibly establish. If you've run at higher lighting for at least that long with no benefit, I'd say insufficient lighting is not the problem; or perhaps the overall plant mass and growth is insufficient reach the tipping point for a new balance even at the higher light.
There is always the possibility that your sand and/or rocks are releasing a massive amount of silicates. But changing that is a last resort.
So that this point, there are no easy answers. If it were my tank, I'd get a bit experimental, changing multiple parameters at once:
1) Increase surface agitation if there's even the slightest chance O2 levels are insufficient. You may lose some CO2, but that's ok because...
2) Over a week, gradually cut CO2 levels in half. 30+ ppm isn't an absolute requirement at both low light and high flow. Reducing acidity will slow the rate at which silicates dissolve, if this is indeed an issue.
3) Simultaneously and gradually increase phosphate levels until doubled. The presence of that small bit of GSA can be an indicator of low phosphate. Higher nitrate levels also require higher phosphate levels in my experience, even if the phosphate level is generally considered sufficient for plant growth. Though there isn't a ratio that must be exactly followed, 40:1.8 is a bit extreme.
And then of course wait to see what develops. If there's an improvement, remove the changes one at a time to isolate which was responsible.