Google Arduino aquarium. Here is one hit open aquarium This is a subject that regularly comes up on this forum so a search here might yield more information, too.
Some Uno shields may require wiring 50-51-52-53 on Mega to 10-11-12-13 (match the signal names) on the Uno to have SPI transfers work correctly.
Same for I2C, 20-21 on Mega to SDA/SCL.