Beaming down from the roof
Tabula Rasa Gold is the only type of pool known to spawn and de-spawn without any fishing activity taking place. They are also the only pool that cannot be fished with skill 1. The easiest way to spot a Phased School appearing is a shard of purple light beaming down from the roof of the cave. Once spawned, the pool will only last about 60 seconds - so you should fish the pool as soon as it appears.