In Part 1 of this series on PVC basics we reviewed a brief history of PVC and explained how PVC pipe is sized and matched to the correct fittings. In this installment we’ll take a look at bulkhead fittings and how to size them for use on your tank, water storage containers and more!

What’s a Bulkhead Fitting?

Bulkhead fittings are designed for use as inlets or outlets on liquid holding tanks, which includes aquariums, filter sumps and overflows. This fitting provides a leak-proof way of attaching PVC plumbing through glass, acrylic and other materials. The standard bulkhead fitting consists of a threaded ABS flanged plastic body, rubber gasket, and an ABS locking nut. The flanged body is often referred to as the front side. The threaded end where the nut is attached is the rear side.

diagram of how to install bulkhead fittings in a drilled hole

A hole is drilled through the wall of a tank or other water-holding container. The threaded body, along with the gasket is inserted into the hole. The flange and gasket can be on the “wet” (submerged) side or the dry side, depending on the application. The locking nut is tightened to pull the flanged body and gasket up against the wall of the container. This creates a water-proof seal without the need for silicone or other sealers.

Sizing Bulkhead Fittings

diagram of dimensions of bulkhead fittings to consider when installing them

common bulkhead fittings dimensions

Don’t make the mistake of thinking a one-inch bulkhead fitting requires a one-inch hole. Bulkhead fittings are identified by the size of the pipe it connects to. The hole size will be larger. The size hole required will be specified in the description for each bulkhead fitting, but is also mentioned in the list above. Bulkhead fittings come in several configurations.

  • Front slip-Rear threaded
  • Front threaded-Rear slip
  • Front and Rear side slip
  • Front & Rear threaded

Choosing the Right Bulkhead

Since plastic bulkhead fittings come in a variety of sizes and four different configurations, its important to select the right fitting size. The first step is to identify the size of pipe and fittings you’re working with. The bulkhead should be the same size. The next consideration is the type of connections you need on each side of the bulkhead. Here’s a common application: Connecting an external return pump to a filter sump.

  1. Match the water pump intake diameter to the bulkhead fitting. Never use a smaller diameter fitting or pipe. It will restrict the pump’s flow rate and may cause cavitation*. [*Cavitation is the formation of bubbles around the pump impeller. When each of the tiny bubbles burst, it creates a high energy shock wave that can damage the impeller. Cavitation will also supersaturate the water with atmospheric gasses like nitrogen. This can cause “gas bubble disease” in fish, a stressful and sometimes life-threatening condition.]
  2. The flange side of the bulkhead, with a slip connection, is often installed on the wet side of the sump. This allows for easy installation of a removable intake screen or 90-degree elbow. By using a slip fitting in the sump, you’ll be able to remove the intake screen for cleaning or point the elbow down toward the base of the sump.
  3. The water pump can be hard-piped into the bulkhead with PVC pipe or flexible tubing and hose barbs. You can glue the PVC fittings or use threaded components, depending on the type of bulkhead and the needs of the project.

Using Uniseal® Fittings

Uniseal fitting

Uniseal® fittings are one-piece rubber grommets that create a water-tight seal much like a plastic bulkhead fitting. Here’s how they work. A hole is cut through the side of the tank. The Uniseal® is inserted into the hole. It has an inner and outer lip and a groove in the middle. Next, a piece of PVC pipe is inserted into the seal. The pipe needs to be lubricated with soapy detergent otherwise you won’t be able to push the pipe through the seal. Uniseals® have a special Dupont Alcryn® wall that expands when the pipe is first inserted, then clamps down, forming a water-tight seal. Uniseal® fittings have proven to be reliable in industrial and aquacultural applications. Many aquarists like to use them on round barrels because the seal conforms to the curved surface. They’re great for making RO water storage containers and salt mixing barrels.

Installing Bulkhead Fittings

As we’ve explained a 1-inch bulkhead accepts 1-inch PVC pipe and fittings. The hole size required to install the bulkhead fitting is larger. You’ll have to drill a 1-3/4” hole through the tank for the fitting installation. Each fitting has its own installation size. If the hole is larger, chances are you won’t get a good seal and the fitting will leak. Uniseals® also have specific hole sizes for proper installation.


Before installing an ABS bulkhead, examine the gasket and flange. There should be no burrs or extra material on the gasket or flange.

  • Make sure the surface of the tank is also free of burrs from drilling the hole.
  • The rubber gasket sits against the flange, not on the nut side.
  • Do not use Teflon tape on the threaded rear side.
  • Hand-tighten the locking nut against the tank. Use a wrench only if you can’t reach the nut.
  • Snug the nut then give about a ¼ turn. Over-tightening can crack the nut and cause leaks.

There are many applications of plastic and Uniseals® in overflows, sumps, refugiums, contactors and protein skimmers. For a DIYer there are endless possibilities to modify existing filtration systems and building your own design. If you’re not familiar with drilling through glass, acrylic and other plastics, look online for instructional videos explain what tools are needed and how to do it safely. In Part 3, we’ll take a look at ball valves and explain how to use them on your aquarium.

Read the rest of the 4-part series, here: