Yesterday's post, it turned out, was premature.
I awakened this morning to find that either Lindsay Buckingfish or Stevie Fishnick had opened a sushi bar during the night. All of their eggs were gone.
It turns out, angelfish will only eat their own eggs or young for two reasons: they are starving (which thankfully was not the case here) or they detect something is wrong. Either the eggs are malformed, they are showing signs of fungus, or the temperature has changed or something else in the environment has altered the healthy growth of their offspring.
This was the first time they had laid eggs on the arch. Not knowing if this triggered an unhealthy growth (or non-growth) of eggs, I kept the arch in the tank but added their red amazon plant. They have been scoping both out today and I suspect they will lay eggs again soon. Hopefully, these will mature and become another clutch of healthy angels.