Slip X Slip Bulkhead Fittings are bulkheads that have a smooth interior for gluing pvc pipe into. Slip bulkhead fitting are the most commonly used in the aquarium industry. The most common use is for the connection from an internal overflow box to an in-stand sump. Usage of a Drain Connector Kit is necessary for the connection. The 1 inch slip x slip bulkhead is the most used size for drain kits. (Water from the aquarium to the filter). The one- and one-half and two inch bulkheads are utilized in larger aquarium systems to handle more water flow. Three quarter inch bulkheads are industry standard for water return from the sump back to the overflow / aquarium.