"glub " is a word, and it usually describes water-noises. It is an onomatopoeic word (the word sounds like what it describes) and not all onomatopoeic words are in the dictionary. Another point is that it is actually birds who say "tweet tweet". Mice and some other rodents say "squeak squeak" or "eek eek" , and if someone sees a mouse sometimes they also scream "eeek ! A mouse!" Cows say "moo", dogs say "woof" or "bow wow" cats say "meow or mew" ." roar " and "tweet" are also used as verbs , that is probably why they are included in dictionaries.

glub, glub is onomatopoeia - like 'woof woof' for a dog. Although technically, I am not sure you can hear a fish so I think glub glub is based more on assinging some imagined sound to the motion of the fish's mouth opening and closing. Roar for lion isn't really onomatopoeia - we use the word for other things, including the human voice if it is a loud, deep, angry sound. Birds tweet - we don't usually use it to describe a mouse - I think 'squeak' is more common.

You are correct it is the sound the fish makes so it is not considered a word.