There's already been a couple threads about this.
this Kanji is known as oh. Dai, tai and a few more are the other pronouncations you can use depending. But when reading the Kanji SOLO/ by itself (with no other kanji)
it is read as oh or dai ONLY.
The pronouncation of tai using just the kanji (no compound word) has no meaning at all. The kanji is read as tai only if it is a compound word, and that is also depending.
Taiyoukai wouldnt mean fish at all if youre using this kanji
... but the pronounciation tai here HAS NO MEANING as a solo kanji, so it means nothing at all.
In compound words... it just depends on the word and context of the sentence. Japanese people learn which way to pronounce it as they learn the compound words.
<-- Dai Youkai.
Youkai (the last 2 Kanji) is already a compound word. The Dai, is acting as an adjective. So it is read by itself. Hence Dai youkai.
You could possibly read it as Ohyoukai/Oyoukai. ( but seriously please dont lol )
The kanji means big; large, but if you want to give off the connotation of something massive or of great power (as in the manga)... Dai is the pronounciation youre shooting for. Since for dai, it also includes the definition great, where as Oh doesnt.
Dai means big; large; Great; loud.
Youkai means ghost; apparition; phantom; spectre; specter; demon; monster; goblin