What is the 'evident pun' in Moonchild?

Recently, a friend sent me this passage from Aleister Crowley's 1917 novel Moonchild:

“Dinner was served; the Poltergeist supplied the conversation. Never before had he been so light, so genial, so anxious to assure us of our future in Summerland; but ever and anon he touched the minor chord, spoke darkly of 'proof,' and of fish! (I beg you all to bear witness that I have not degraded myself by the evident pun)."

(emphasis mine)

We both understood this as the author saying that the preceding sentence contains an obvious and enticing opportunity for a groan-worthy pun, but that he restrained himself from actually making it. But we can't for the life of us figure out what the pun would have been.

(I considered posing this question on scifi.stackexchange because it's a fantasy novel of sorts, but ultimately chose english.stackexchange because the question is purely linguistic and (presumably) requires no further context from the story beyond the one paragraph already quoted)

The Poltergeist spoke darkly of 'proof'.

The alcohol content of liquor is described using the term proof (50-proof liquor is 25% alcohol).

Another word for hard liquor is spirits.

A poltergeist is a kind of ghost, and another word for a ghost is a spirit.

I think the narrator is being facetious in suggesting that the pun is so obvious. Rather, he's challenging the reader to figure out this chain of synonyms.