I find using T5 for cheato growing is quite good.
I think a lack of nutrients might cause them to die off as well, since ya mention ya have rowa running as well and undetectable nitrate.
Didn't dose much iron for my case.
Its just a marine plant we used to export nutrients such as nitrate and phosphate.
Normally, we put a clump in the sump, shine some light on it[10 hrs for me] and harvest it every month.
google cheato + reef central for more info.
Bro underwater is correct, juz aim the wavemaker at the bottom, then u will see the detritus gather at one corner.
Me rather lazy to siphon at times, heheheheeeeee.