In response to Worton, I'm just using a standard sponge filter (the a sponge going horizontally along the bottom of the tank with an uplift tube) and a moderate air pump.
My pair have spawned again, but it's not all good news as the female appears to be going blind- she's got two white circles around the lens in both eyes

. I was a bit dubious about the water quality, but testing suggests it's uncannily like most of my other tanks- 25-50 ppm nitrate, no ammo or nitrite, pH c. 6.5, fairly hard on GH but very low on KH (or the opposite- I never can remember which my water's hard for!).
I find it impossible to see if there are any eggs on the fish food lid when there are bubbles on it without lifting a corner of the lid up slightly, which makes the bubbles disappear if they're not fresh! I only checked this time as they were due a water change, and the male's been playing with the lid a bit, shoving it round the tank like he's playing trains. It's really surprised me that he's done this- I was expecting him to regard the nest as a fixed point, but it definitely appears to be deliberate how he moves it if it drifts into an inappropriate spot. Fortunately he quickly repaired his nest- now it looks like a proper bubble nest, with lots of small, sticky bubbles rather than just one big, non-sticky bubble under the lid.
There were a few small endlers I'd left in the tank from previously- I removed all bar one of these (which evaded capture). It shows how small male endlers are because my male glowlight tetras started trying to catch them in the tank I moved them to.
Trying to catch the female failed as the male belatedly started showing strong defensive behaviour, barrelling into the net each time I tried to put it in the tank. I need to move the remaining endler, plus I suspect both parents (I don't know how old the eggs are- possibly several days), but have a problem at the moment as my other spare tank has had a heater failure- according to the CateLog hoplos can cope with it at room temperature, but I don't want to shock the pair, especially when one's already under the weather.