I'm not familiar with the exact systems in use here. but I work with marine AC and refrigeration regularly.
I would think that the fish box plates and the AC are 2 separate systems. this is because a single system cannot produce both freezing temperatures for the fish box, and above freezing temp for the AC.
so, what we should have is a low temperature system for the fish box, most likely a small DC compressor running r134a with an air cooled condensor. and a separate AC powered compressor running r410a with water cooled condensor/heat exchanger.
so if the fish box is not getting cold. I would check in the following order.
1. check to see if the compressor is turning on.
a. if yes move to 2
b. if no, check thermostat continuity. if thermostat is good, check controller
2. check is condensor has a cooling fan. is it working? if yes then check charge of the system.
let me know if I can help more.