Pool light staining and putting brown gunk into the water

[jefski]

When we bought our place in May 2019, the pool light was not working. It would have been installed in 2013. It was a Pentair Amerlite and Amerquartz light niche with an Intellibrite controller.
In May 2020, we made several changes: converted to SWG, installed a variable speed motor/control, changed out the filter sand with glass, and installed a new Intellibrite 5G color led light along with a new Intellibrite controller. The owner of a local pool supply shop has been doing all of our work (until the last week or so, when I started doing my own testing and treatment, thanks to all of you!).
It wasn't long until the shiny parts of the light started turning brown. It looks kind of like rust, but I don't think that's it. If scrubbed enough with a blue woven pad, the shine comes back with no pitting (not the whole thing, some of the stains have resisted all of my scrubbing). And filmy, slimy bits of brown come from the light and float in the pool.
A month or so after the install, we had an inline anode added, per Pentair's recommendation. It's between the pump and the filter. The anode is visible in a clear case, and it's actively anodizing, but the light didn't improve, at least not enough to really tell. I cleaned it over the winter with muriatic acid, so it was fresh again this spring.
Over the winter (with the pool closed), the light got significantly worse. When we had the pool opened this spring, our pool man got Pentair to send a warranty replacement light. A large amount of the brown gunk came out from behind it when it was changed. The new light immediately started turning brown. Looking at it more closely, I don't think it's originating on the front, but coming from behind. The staining and discoloration is notably darker and more scrub-resistant where water is able to move between the front and the back. So far this summer, we don't have any response from Pentair. Because of the effect on our pool water (which may have been due to poor chemistry maintenance, but naturally we're blaming the light), we're ready to have the light yanked and a cover put in its place. The attached image is from last summer, and after a scrubbing. The debris below gives an idea. It looks quite a bit worse now.
Does anyone have a better suggestion?
Thanks for reading and for all the help you've already provided in many other posts.
Jeff

 

[ajw22]

Did your pool store testing test your water for iron and copper?

 

[jefski]

Yes, both are zero and usually are. Sometimes we get copper after refilling or over the winter, but it doesn't stay long.

 

[JamesW]

Can you pull the light up and show pictures of the whole light from every angle?

Can you show pictures of the inside of the light niche?

You might have some sort of stray current causing accelerated corrosion.

Has anyone noticed any electrical issues or felt any shocks especially in cuts?

 

[JamesW]

Is everything bonded?

Is anything getting excessive or noticeable corrosion?

The aluminum on the automatic cover is at high risk for accelerated corrosion if there is a stray current.

 

[JamesW]

Can you show pictures of the system?

Is the electrical all good with no issues?

 

[jefski]

Can you pull the light up and show pictures of the whole light from every angle?

Can you show pictures of the inside of the light niche?

You might have some sort of stray current causing accelerated corrosion.

Has anyone noticed any electrical issues or felt any shocks especially in cuts?

I'll see if I can get the light out for more pics.

No known electrical issues. The panel and wiring seems functional, if perhaps not the tidiest I've seen. Bonding is unknown. The installation doc shows the ground lug on the outside of the niche where we'd have to dismantle the pool cover to access it.

No other metal parts are in normal contact with water (except for splashing). No other noticeable corrosion.

 

[jefski]

Checked the grounding between the panel, pump motor, and heater. Zero ohms resistance between each of them and the earth ground. The cable to the light is grounded properly at the panel.
My wife took over making follow up calls and she just made a breakthrough this morning, we should see a service man from the Pentair dealer in the next week or so. Should have given this to her long ago!
Thanks for the help.. Will post again after the visit. Their initial reaction is the same, that there's an electrical issue.

 

[JamesW]

Checked the grounding between the panel, pump motor, and heater. Zero ohms resistance between each of them and the earth ground.

Check for voltage and current.

The next time someone has a cut or scratch on their finger, stand on the concrete on a wet spot and put the cut under water to see if you feel anything.

 

[jefski]

Talked to a service representative from Pentair yesterday. He suggested 2 things to check. (1) get an electrician to look for stray current and (2) see if the slimy gunk is white water mold. He experienced the same gunk behind his own pool light.

After reading several posts on TFP last night, we decided to follow the mold first. I just started with TFP about 3 weeks ago, and now I think that our FC levels had never been high enough (rarely much above 2, and I was told it's normal to have zero in sunny weather). I've been watching the CC levels daily now and seeing them move up and down between 0.2 - 0.6, with nothing (other than the brown gunk) that should have caused any increases. Been adding a quart or more of 12.5% bleach daily.

So today, we pulled the light out. Everything behind it was covered by the film. We scrubbed it out and vacuumed most to waste. We cleaned up the light and reinstalled. The inside of the niche came pretty clean. The back of the light still has some blackish patchy staining. Rust remover and scrubbing got most of the stain off the front, but we may have dulled the chrome-like finish in a few places. Even the glass had some obstinate brown staining at the bottom but some cleanser and scrubbing finally got it.

Bumped our FC to about 7, and now we'll watch a bit to see what happens. One issue is the sealant where the cable goes through the back of the niche. It was loose and crumbling (just installed 2.5 months ago). What do you recommend for a sealant?

Per an earlier question, there was no sensation when putting a cut finger in the water, even firmly and wetly touching the concrete deck.

From the other posts about white water mold, it seems that FC is sufficient to control it. But with little water flow behind the light, is there anything out of the ordinary we should do? I wondered about simply using the light more, as the heat should cause at least a little convection flow. Or even a chlorine tablet placed in there?