Sealing light casing hole?

[Shpigford]

So I've had pretty major water loss (1-2" per day) since we got our pool up and running this year (bought the house, had to replace the liner) and after doing a bit of investigating, I found that the hole in the back of our underwater light casings (we have two lights) where the cord for the light goes out of the casing, is just wide open. Nothing sealing them at all.

Am I correct in assuming that's probably a major cause (if not the cause) of our water loss?

And if so, what do I seal the whole at the back of the casing with?

 

[PoolGuyNJ]

The hole, on the back of niche has a conduit leading to the deck box. It should be water tight already. While it will fill with water, it shouldn't leak. A squirt from a dye pen will tell you if the conduit has been compromised. If it has, a couple wraps of butyl tape around the light cord will seal it and still be removable, should the fixture ever need to be removed.

A professional diver can inspect the pool if a pressure test hasn't revealed anything.

Scott