It is a shelf I made from aquarium grid. I use the grid for climbing structures in the tank! Here is the one I buy from Amazon. :)

https://www.amazon.com/OBANGONG-Aquarium-Divider-Segregation-Breeding/dp/B08S3K1B3F/ref=sr\_1\_6?crid=3QB2K6QAQFL5A&keywords=aquarium+grid&qid=1650933331&sprefix=aquarium+grid%2Caps%2C317&sr=8-6