This morning’s test results were:

• KH: 10 ° (179.0 ppm)
• ammonia: 2-3 ppm
• nitrite: 0.5 ppm
• nitrate: 5 ppm

So the KH really is dropping, and I have now remembered that the reason it rose was not the sand, but that I used tap water to top up! Nitrite and nitrate are definitely rising.