Bulkhead components

Bulkhead fittings are designed for use as inlets or outlets on liquid holding tanks, which include 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 or PVC flanged plastic body, rubber gasket, and a 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.

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 waterproof seal without the need for silicone or other sealers.


Bulkheads explained

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

common bulkhead fittings dimensions


Choosing the Right Bulkhead

Since plastic bulkhead fittings come in a variety of sizes and four different configurations, it's important to select the right one. 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.

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.  Diamond-coated hole-saw bits are best used for drilling glass so just choose the bit based on the hole size you need for the bulkhead.  Most bulkheads have a little wiggle room in terms of the exact hole size but always be sure it's within the noted range for that particular bulkhead. 

  1. Drill the right size hole into the aquarium or container
  2. Push the rubber gasket onto the bulkhead, over the threaded side.
  3. Insert the front flanged side of the bulkhead through the hole.
  4. Attach retaining nut and tighten. DO NOT OVERTIGHTEN, you can crack the bulkhead. Generally hand tight is sufficient.  
  5. Proceed to attach PVC or plumbing fittings.
  6. Perform water tests to ensure a water-tight seal.

**Pro-Tip - 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.

Example: Connect an external return pump to a 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 is designed to be mounted on the inside or "wet side" of the sump or container. This allows for a water-tight seal and easy installation of a removable intake screen or 90-degree elbow. By using a slip fitting in the sump (without glue), 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.

Uniseal Fittings


Uniseal fitting

Uniseal® fittings are one-piece rubber grommets that create a water-tight seal much like a plastic bulkhead fitting except they are slightly flexible. A hole is cut through the side of the tank and the appropriate 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.

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.

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