When it comes to adding magnets to your fish tank, the placement is important. Knowing which side of the magnet goes in the aquarium can help keep you and your fish safe. Generally speaking, when putting a magnet into an aquarium, always put the smooth side facing outwards and away from any living creatures that may be present in your aquatic environment. A rough or jagged surface could potentially cause serious harm if they come into contact with fins and scales as some magnets have sharp edges. Additionally, keeping this smooth side out will also reduce algae build up on the outside of the tank as well as prevent corals or other objects from sticking to it.