Are they individually sentient or more as a group/mob sentience? Individually, it may be a crazy mixed up fish world, but maybe if they were more in tuned together like a hive/school type mind it would be easier to put together. Help explain how they travel, maybe suddenly the entire school of them would turn on a few weak like they finally determined were holding back the community, they all identified a threat and as a group would suddenly turn and descend on it etc. Actually, maybe I am just thinking kinda of Little Nemo and getting directions. Hmmm. Like it though, but maybe a hive mind with different voices answering depending on the question. Nemo school of fish with their own agenda and strength to carry out could be dominating if they chose to do something crazy.