I was just about to add 1 gallon of muriatic acid for every 2500 gallons of water to perform an acid start-up of a pool. It just occurred to me that maybe I should remove the lights from the pool.

Will this concentration of muriatic acid for 3 or 4 days damage the lights?

May not be a bad idea. I remember reading somewhere that it can damage them. I don't pour MA near them for that reason. Maybe someone else can offer more help.

Most of the opinions I have received locally are that no damage will be done over the course of 3 - 5 days so I left them in. If it turns out I was wrong, I will amend this post with the information.

I know this post is old, but, yes it does ruin the faceplate of the light. It will turn it brown and will not come back to the original luster. Also, dillute acid in a bucket of water before tossing into the pool. do it at a rate of a 1/4 gal to 4 gals of water in a bucket

But if you have garden hoses that have grown that impossible-to-remove black mold, chuck them in overnight. (I kept the brass fittings just above the water line, not sure if I needed to.). I discovered this by accident when the pool service crew left a small loop of hose hanging in the water when they were washing off the coping after pouring acid. I pulled it out the next day and it looked like new. Then I did the other 250' I had lying around.

We did not dilute--we just dumped 3 5-gallon carboys of MA straight in: one at each of the long sides plus the deep end. No problems with a white plaster pool. We did pull the light and we put a cover pump in the deep end on top of the main drain running 24/7 to keep the acid distributed.