I have switched to a Hanna digital pH tester, (calibrated recently), so I am fairly sure the pH is at least close to accurate....

I ended up doing a big substrate clean. I lost my one adult male, and couldn't see where he had gone at all. I have always used a vacuum nudging at the media, but when I started moving this stuff more vigorously I was shocked by the amount of crud that was hiding towards the bottom of the tank. I guess the Biube's central under gravel filter is not as good as I thought it was.

So, I have done my 10 litre water change for the day, cleaned all the crud away, (which I am guessing is the underlying cause of the problem?), and have found another issue.....

.... the Biube has a hexagonal nut at the bottom of the tank screwed into the airline intake. I noticed that it was leaking, (and by the small amount of crusted deposits around the airline had been for a little while). It looked to be coming from between the nut and the screw, so I have tightened it. I had to use pliers and was a little worried since both parts are plastic, but its either stopped or slowed to a crawl now.....I will check it in the morning.

Do you think I have found the answer? I would like to think that this is it, (although it means I have caused this myself, by failing to keep the substrate clean). I am crossing my fingers that the remaining fish will survive long enough for the water to be brought back to safe pH levels, they don't seem as stressed, but it may be wishful thinking.

Under gravel filters are less efficient and they need regular maintenance to remove excess detritus from below the plate as well as removal of accumulation in and on the substrate. This involves taking the whole thing apart every so often to remove the plate, or ingenious use of a powered syphon to suck it out. Using just a UGF means generally light stocking to try and limit the accumulation. The Biorb UGFs are known to be very inefficient too, so understocking in any of their tanks is the best way to go.

It sounds like this is the cause of the rapid acidification of the water, but bear in mind the crud you can see isn't necessarily the whole story: there may be a lot more underneath.