It only happens from time to time after I vac or switch the pump speed from low to high and even then not every time. I thought I had an issue with my filter last year and had the pool company come check it under warranty. They thought maybe it was letting some of the sand back through, but they dumped the sand and checked everything and replaced the sand. They couldn't find anything wrong and we couldn't get the problem to replicate while they were there. So I assumed everything was ok, I thought maybe I just get a lot of dirt from high winds here in Oklahoma. Then this year I open up to really clean pool. Water was clear with a little bit of dirt on the bottom. This weekend I was running the vac and was about ready to pull it out, so I shut the pump off. I then decided it had missed a couple of spots so I was going to manually hit those. When I turned the pump on I noticed the water was visibly darker for a split second coming out of the return. The spots I had missed were on the other side of the pool and I know the area around the return was clean before I moved the vac. After I got the vac out I noticed a couple of spots on the pool floor close to the return. I don't know what to do. Is there some other explanation for this? Please offer any advice you can. Thanks,