Geezus that is alot of light for a 60cm, I use 2x36 on mine with it 6" to 10" above the tank at various intervals and it is still a lot of light.
I run one for 8hrs and two for maybe 2 to 4hrs on some days, not everyday.
But moss is much more "forgiving" than the plants I am curently growing in my 60cm.
I have an AHS reflector which adds punch to the lights already, are you useing the AHS reflec's?
Just play around with it, you can tell when to much is being burned, BBA will start, instantly back off some, then you will have to trim out the BBA, it won't just vanish, and add a shot of Excel.
This is how you find that threashold.
1x55 would be plenty if you could adjust it up and down.
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