The Fluval A66 fits the following:
The Fluval A76 Elite Foam Filter for A70 - 5-pack includes five replacement aquarium filters for the Elite A70. The Elite aquarium filter removes debris in aquarium water and encourages the growth of beneficial bacteria populations. This foam aquarium filter provides very efficient mechanical filtration for your aquarium.
Fluval A76 Elite Foam Filter for A70 - 5-pack specifications: