Added note to
>>172:
According to my wife (No. she is real, Japanese, and has a bachelor's degree in Literature and teaching qualification in junior high Japanese class), the rules are explained in detail here:
http://www.aozora.gr.jp/cards/000061/files/377_2754.html
Note that the article is an uber-wall of text in Japanese (I gave up after the first few lines).
Today I happen to be staying at her parent's house and had a chance to ask them the pronunciation of 水族館 (sui-zoku-kan, an aquarium), which consists of three on-yomi kanji but seems to satisfy the simple condition at the 族館 part. Their answers are:
Her parents: suizokkan.
Her sister: The pronunciation is suizokkan, but you write it as すいぞくかん (suizokukan) in hiragana.
My wife: Finish your lunch already!