Stage 1 of Operation Shady is complete.
Will take a photo when water has cleared. Tank looks more conventional now with wood semi-buried in the sand and moved to the middle-back. Part of the tunnel widened and placement is now parallel to sides of tank so you can see right through. Plants pushed to the edges more, some stones removed. Foreground is now mostly bare sand for swimming, with some bits at the far sides only.
Fish will be under observation to see if behaviour changes. Snails have begun to take over, am thinking of getting them a dwarf chain loach as housekeeper.
A fishy miracle occurred - fishy because it could have just been a fluke and fishy because it involved a fish.
I took the wood out and gave it a shake to give the fish a chance to come out. It seemed they had, I had a good look inside. I set the wood down out of the water and was fiddling with the plants. After a few minutes, possibly up to 10 mins I took the wood down to the garage to see about breaking it up or widening the hole. I put the secateurs in the hole and clipped a little bit out. Set the wood down, pondered which saw to borrow. Came back to the wood and saw a fish inside on it's side where previously there hadn't been. I thought it was a goner and was about to carry on. Changed my mind, thought I had better at least dislodge it before continuing. Took wood back to the house, put it in the tank, fish disappeared. Rolled the wood around and around in the water shaking it, couldn't see the fish. No dead fish in the tank either. Took wood out, set about catching the fish to count them - lo and behold 10 live fish in the tank.
Is that weird or what?