Kuragehime

Amamizukan is an apartment complex where no boys are allowed. Kurashita Tsukimi, a girl who adores jellyfish, lives there happily with her friends who all have nerdy obsessions of their own. Their peaceful lives gradually start to change when a beautiful woman helps Tsukimi out of a pinch... but it turns out "she" is really a "he."

(Source: Anime News Network)