How to repair a small plastic tank?

you can use it on plastic as well as glass, just make sure the surface is clean, properly clean, so that the sealant has something to key to. youd be better off replacing the plastic, or even just buy another tank and thieve the bits off that one, 3 gallons arent expensive at all. id ditch the filter media and start again, purely because you dont know if there were any diseases e.t.c. in your cousins tank.