Woke up to the tank making some noise last night at about 2am.
The level in the display was rising and then "flushing" down as the secondary drain achieved full siphon and drained the high level, and then repeated about every minute or two.
The main drain was clogged at the gate valve, and so no water was making it through that pipe.
I opened the gate valve and not even a full turn and it starts flowing well again, to my surprise a fish pops out of the pipe.
Turns out my diamond goby had jumped into the overflow and managed to get himself sucked down the main drain and stuck in the gate valve. I don't know how long it was stuck in there but he seemed a little shocked in the sump. I fished him out with the net and dropped him back in the display.
He was still ok this morning when I left for work, fingers crossed he makes it through the day.
They are known jumpers, and I hear this one splash around from time to time. I haven't pursued a lid though because the canopy I use on the top would prevent it from actually getting out onto the floor.
Looking into screen tops now, but fitting something to the shape of this tank is going to be tricky.