I've tried to get my wife to leave blinds shut as well and she really dislikes that. I was at first concerned all the light hitting my empty tank may give rise to a harsh reflection here and there or maybe even a hazardous shine but I survived that phase in-tact.

With the tank running now I intend to shut them whenever she's not paying attention.

So it does sort of sort of sound like the consensus here is that even if there were phosphates in the water column, rarely are the test kits able to pick up on it. The difference being either by interference maybe that whole organic vs inorganic phosphate question, who knows.

FWIW I let the sun hit my tank as often as possible (really only possible in the winter though, the rays come in at the wrong angle from a few weeks outside of the equinoxes). The clams in particular, love that!

The "I have never had a PO4 reading" option is quite irrelevant because I doubt anyone who voted for that option used a D-D Merc kit or another high sensitivity kit. A "zero" on a Salifert or Elos kit is not actually zero, it is simply "undetectable" with those low sensitivity kits. Even if you got a "zero" on a D-D kit, it is highly unlikely that it really is zero, it is also "undetectable" with the D-D kit. PO4 can also be taken up by algae in such amounts that there is little left in the water column to test at any given time.

Precisely. If there is more phosphate in the water column than the plants (both "good" and "bad") and animals can take up then it becomes a poison of sorts. We don't like nuisance algae and cyanobacteria so we try to lower phosphate to the level that our plants and animals can handle. We have many methods of lowering phosphate; GFO, carbon sourcing (VSV, Fauna Marin, Zeo, etc), and competitive elimination (chaeto).

Your test kit (any kit) will only read the amount of phosphate in the water column that exceeds the demands of all the organisms in the tank. If you have nuisance algae, cyanobacteria, and/or chaeto growing in your system this expresses the PO4 difference between the kit's reading and the demands of the "good" organisms in the system. If your test kit is "not detecting" phosphate, and you have nuisance algae you do have an excess of phosphate.

Nitrate also plays a role, but we are discussing phosphate here. The two are usually quite intertwined, and in most discussions aren't of significance to differentiate.

Personally, I gave up my GFO. GFO has never been in my new tank. So I voted:
"I run a ULNS" [Zeovit, just starting the nutrient lowering full system]
"I test PO4 frequently" [with Salifert it is "undetectable", and has been so since shortly after the tank finished cycling]

I used a D-D kit and a colorimeter and voted that option. :p

In your opinion what is the difference between "undetectable" and "not present at this level" ? To me "undetectable" simply means "present in levels below the threshold for the reading category". In the case of D-D/Merck, the lowest value is 0 to 0.008 (which is VERY low IMO - as in, less than 0.01 - no other test kit, including the colorimeters can read to that low of resolution). One thing about the D-D/Merck though is that it is not a linear scale.

Hey Tony, I added some more thoughts to my post before I saw your reply. I'm bad for editing, and re-editing, and editing some more adding more thoughts. :D

Imo, the difference between "undetectable" and "not present at this level" is that "undetectable" admits the test kits' accuracy may be (probably is) lacking where "not present at this level" suggests a positivity that we really can't confirm.