I do leak detection on swimming pools. Just because the water doesn't go down to the light doesn't mean the leak isn't in the light. There is a conduit, usually brass, that comes out of the back of the light. If this conduit has a joint in it before it turns up into a junction box it could be leaking there. This is what I normally tell people before I come out to dive on their pools. Do a bucket test: Set a bucket, preferably a transparent one, and set it on a step and weight it down. fill it with water to where the water line is the same on the inside and outside of the bucket. Let the equipment run it's normal cycle for 24 hours. Check the difference in water levels between the bucket and pool. Fill it to where it is equal again. Turn the pump off for 24 hours (don't do this if there is a possible freeze). Check the level again. This will help tell you if the water is leaking from plumbing or structure. Many times I have seen pool owners convinced that they know where the pool is leaking but we find out it is something totally different. Good luck.

It certainly sounds like a small leak. Plaster is put on wet and 12 days is a long time to blame the plaster. But I use to live in Northern Ca and have family and friends there and it was 100 degrees on the weekend. Was anybody swimming in it....pretty normal to loose an inch of water from kids swimming and jumping. Was it leaking before it was replastered? Before they replastered did they do a pre-plaster prep and reseal around all the pipes? Did you check your equipment pad and see any places where some older equipment that might have been sitting for awhile unrun might be leaking around seals onto the pad? Those are some obvious things. After that, the only way is to complain and tell them to send a diver over to check for leaks. I don't know about your plaster contract but mine said they aren't responsible for leaks.

Shhhh...we thought about that too. Yes the filter is 20 years old. We took the grid assembly apart last night and methodically cleaned every single piece even though nothing was really dirty to the eye but obviously DE was leaking thru something. The grids were all perfect. We even held them up to the sun to look for small holes. Put it back together and not one piece of DE so far. Let it run all night for the first time since opening last week and all is still well. Shocked it heavily this morning. I am cautiously breathing easier.

The first step is to remove the grids from the filter and inspect them. What brand of filter do you have?