A safety system for a fish-tank siphon is an apparatus used to prevent suction of a fish-tank-cleaning siphon from becoming obstructed by fish, underwater plant life, rocks, or other large tank contaminants. The apparatus is configured to protect the intake by preventing potential obstructions from entering the siphon at all. The apparatus includes a grate, a mating mechanism, and a siphon inlet. The grate acts as a protective filter, preventing large items from entering a connected fish-tank-cleaning siphon. The mating mechanism allows for connection of the present invention to a fish-tank-cleaning siphon. The siphon inlet is a section of the fish-tank-cleaning siphon through which the vacuum is applied in order to remove dirty water and water contaminants. The general configuration of these components allows the apparatus to prevent a fish-tank-cleaning siphon from incidentally collecting fish or other underwater creatures, plant life, pebbles, or other contaminants while vacuum-cleaning a fish tank.