15% wc a week, should not be any detritus at all.
Well, yes and no. 15% is great but it doesn’t necessarily mean there’s no detritus building up. If you only change the water but don’t stir up/vacuum the sand bed then you’re not getting any of the detritus that is down there.
Have you had any other deaths? Fish, inverts...? Or just corals. I agree with
@Vhuang168 about the icp test. The lab will test so many more elements than you can test at home, plus if one of your test kits is bad (or you’re even doing it wrong

), you may never know. Even the more expensive ones are like $60 which is still cheaper than a new light.
I think people tend to blame lights because there’s more of a “mystery” with them. Not that there’s not scientific means of measurement with them but we as hobbiests don’t have the ability to really test those properties on our tanks. Sure there are PAR meters but not that many people actually are able to buy or borrow a meter to be able to test it. Do you know how much PAR you're tank is getting? What about PUR? Then there's the spectrum. Is the light causing more chlorophyll A or B to photosynthesize. There are just a lot of variable that are either unknown or not understood (myself included), so people blame it. I think water is a bit easier to for but things can simply be overlooked. In general, it's much easier for the water to be the problem than the light.