The Laguna PT550 fits the following underwater filters:
The Laguna PT550 PowerFlo Replacement Foam 2-Pack is a replacement foam filter for the Laguna PT500 Powerful Mechanical and Laguna PT505 Biological Underwater Filters. Arriving in a convenient two pack, this foam aquarium filter provides mechanical and biological filtration. The PowerFlo filter efficient captures dirt and other debris in your pond water.
Laguna PT550 PowerFlo Replacement Foam 2-Pack specifications: