(it's from a comic book, that's why everything is in caps.) i'm trying to understand this sentence, however i don't quite understand in what order i should try arranging the words for it to make sense. thank you!
"Etwas zu lachen haben" literally means "to have something to laugh about". It's mostly used figuratively, meaning that somebody has some joy in their lives. It's often used in the negated form, "Jemand hat nichts zu lachen". That can mean that somebody is living in bad circumstances or is treated sternly by somebody else. If you messed something up in your job and your boss asks you to come to their office, it could be said about you that at this moment "Du hast nichts zu lachen".
"Ruhig" literally means "quiet", "calm" or "tranquil". Here it's used as a particle. You can read it as a kind of "it's ok for me", "you're welcome to" or "I don't care":
Du kannst ruhig das Fenster aufmachen (das stört mich nicht).
Er kann ruhig herumbrüllen (das ist mir auch egal).
The whole sentence could be translated as "The fishes should have something to laugh about every once in while, too, for all I care". A more natural translation could be "Why shouldn't the fishes have some fun, too?" or "Fish deserve some entertainment, too". There might well be some kind of word play, but without knowing the context that's hard to tell.