Burry the tunnel under your aquarium soil or gravel and let your fish explore it. 

All the edges are rounded to protect you fishes' fins. With dimensions of 140mm x 140mm and a 37mm roof height, (50mm overall height, including the entrance). The entries are 30mm x 40mm. The plastic thickness is 3mm.

Materials: Pick materials that are safe for both your fish and plants. It's usually recommended to use ABS or specific food-safe PLA, and it's often advised to coat the print in a safe epoxy to seal it.