The aquarium gift shop sells small bottles of liquid plant fertiliser designed for home fish tanks. Penelope is setting up a new tank at home and wants to add the fertiliser to help her aquatic plants grow faster. She notices that the aquarium uses a similar fertiliser in their large display tanks.

Penelope: Since the aquarium sells this fertiliser in their gift shop and uses it in their own tanks, it must be safe to add any amount to my home tank without harming my fish.