Chaeto is dying

[gregzz4]

I'm askin' this kinda sheepishly as I feel, with all that I have learned here, I should know the answer to this ...
My chaeto is dying ...
My alk has been low for a week or so, but I'm currently bringing it back up due to the addition of some montis
The Alk was 5 ish and I didn't know

I am assuming my chaeto is dying as there is not enough PO4 for it to live off of ??? and it has nothing to do with the Alk or Ca ?

I've also been told to run my chaeto light 24/7 instead of 12-12
What's your thought on that ?

Thanks for your help ( I think my chaeto is going to die out )
Greg