I can see that this topic has been broached before, but it’s been three years and I can’t see why it’s still like this.
I feel like removing this vocab entry may have been warranted since 海水魚 was added. The first two google results for ‘うみざかな’ are Wanikani forums threads (which I get is possibly because writing it in kana is less common, but I think it still indicates something).
Anyway, I’ve gone back and seen the reading section for this entry and I can understand the sentiment, but I think it’s weird to completely disallow the reading of the word which actually appears in dictionaries. As far as I can tell, no other vocab entry on WK is like that. I’m not suggesting うみざかな be removed as a reading (because I can believe it’s more common in spoken Japanese), but かいぎょ should definitely also be accepted. I will admit I’m making this post because I’m annoyed that I got this ‘wrong’ by typing in a valid reading! When I first ran through WK years ago, I am pretty sure かいぎょ was the only valid reading (or at least it was the main reading).
First things first: did you get 海魚 marked incorrect for entering かいぎょ? Because if so, that shouldn’t be. It should only be warned. I tried it on my end, and I got the warning with no penalty.
As for why that reading is warned now, that was a content team decision by the native speakers on staff, based on not overloading with too much info, and focusing on what is most common and useful to learn. They felt うみざかな is the more common and useful reading, and since users already learn the more technical 海水魚 at level 7, they decided it wasn’t necessary to teach the かいぎょ reading for this item.
Let us know if you are getting marked incorrect for this reading, and we’ll investigate it with you!