Mod Podge is not considered aquarium safe. While Mod Podge is advertised as non-toxic, it contains ingredients such as polyvinyl acetate, polyvinyl alcohol, and other additives that can potentially harm aquatic life. These ingredients can leach into the water and cause health problems for fish and other aquatic organisms, such as discoloration of water, reduced oxygen levels, and skin and gill irritation.
If you are looking for a sealant for your aquarium, it is recommended to use aquarium-safe products specifically designed for this purpose, such as silicone sealant or epoxy resin. These products are made with materials that are safe for aquatic life and will not harm your fish or plants.