Students at Copperbelt University (CBU) lit candles and marched around campus to mourn the big fish.
The hashtag Mafishi, as the fish was affectionately known, is trending on Twitter in the southern African state.
For the past two decades, CBU students have believed the fish would bring them good luck in exams.
Mafishi, meaning “Big Fish” in the local Bemba language, was thought to be at least 22 years old and had lived in the university’s pond for more than 20 years, student leader Lawrence Kasonde said.
His death was still being investigated, added the president of the Copperbelt University Student’s Union.
“It is yet to be buried, we are planning on embalming it,” Mr Kasonde told the BBC.
President Lungu quoted Indian anti-colonial campaigner Mahatma Gandhi in his message of mourning, saying on Facebook that “the greatness of a nation and its moral progress can be judged by the way its animals are treated”.
“I’m glad you received a befitting send-off. We’ll all miss you,” he said.