I've been a pimp for the sand and gravel filters as the most effective, low cost, DIY filters for quite a while, and frequently recommend them for people just getting into the hobby. I've never really had a bit of trouble with my two until this week (or actually I now realize for probably the last year in varying degrees)
The quick version is that since I installed an RDF on the lower pond I've had to backwash the S&G less frequently as is evidenced by the stuff that goes to waste from the S&G's backwash. I also recalled that Birdman left his S&G's for a month when he first went to SoCal. Therefore I've been waiting 2, 3 + weeks between backwash of the filters. What I had no idea was happening was that as my filters were clogging up (and the time they remove ever smaller particles) the water was backing up through my backwash tube and gradually seeping out of my contraption of a air blow backwash manifold you can see. Therefore always cap that air intake tube during operations.
This was a complete mystery to me over the last year because I meter the water on my autofill to regulate my water change. Gradually over a week, or two my autofill would double the water added. I assumed it was the RDF washing more because the pond was looking ever clearer (as the S&G's clogged and filtered ever finer). I also have a LOT of trees so I get large quantities of stuff at random intervals. I've tested every imaginable possibility of a leak because at times the water use was getting excessive. It was an absolute mystery to me as Norm can attest to. I would bounce every imaginable idea off of him and he politely listened and made suggestions. We could never trace the source of the problem.
What became curious was that the water use seemed to always rise on Thursday and Friday. I even suggested cycles of the moon out of frustration. Again, this happened over a year so I knew it was as I neared the weekend that my water use would rise, and I had no explanation. I always backwash on Sundays so this explains the cycle.
It wasn't until this week when I finally let the S&G's go over 3 weeks that the upper pond dropped a lot. I shut off the circulation between the two ponds and it continued to drop. I was absolutely sure I had a liner or BD leak and was crestfallen. I finally went to backwash the filters out of frustration and saw water pouring out of the bleed off valve in my manifold which I leave partially open as it's perfectly set for the bleeding off the right amount of air during backwash.
If I was using my wet vac like I used to it wouldn't be a problem because the backwash tube simply flows back into the filter, but because of my manifold extending out of the barrel area and because it's covered and near the back of my filter area it was simply backing up into the air manifold and seeping out undetectable. I also have a river rock base under the raised barrels with a french drain so I couldn't see any water pooling nor even dampness.
So, lesson learned. Backwash your S&G's weekly and have a way to cap your air tube leading into the bottom of the barrel if it extends beyond the edge of the barrel.
Doh