My most recent fish rack build made me realize how much I don’t know about plumbing. I’m sure that there are others with much more knowledge than myself on this topic, but nevertheless I decided I would share what I’ve learned in the hope it makes someone’s life easier setting up their tank! This is not an exhaustive guide; this is just information I had to learn on my build. If I’ve said something incorrect, which is entirely possible, please don’t hesitate to chime in… I’m still learning this myself! If you've got something to add, chime in as well 
This information is meant to apply to plumbing for aquarium water, not air (some differences exist).
Codes:
All of the fittings and pipes you are going to be playing with are labeled with codes that tell you what they are.
-NPT: National Pipe Thread (Tapered)
-NPTF: National Pipe Thread Fuel (Tapered)
-GHT: Garden Hose Thread (Not tapered)
-Spigot or M: Fitting with threads on the outside of the fitting
-Socket or F: Fitting with threads on the inside of the fitting
-Slip: Fitting with no threads
-PVC: Polyvinyl Chloride plastic pipe
-CPVC: Chlorinated Polyvinyl Chloride plastic pipe
-SCH ##: thickness of the pipe wall
-ID: Inner Diameter
-OD: Outer Diameter
Fittings:
A fitting is used to connect lines. Those lines might be pipes or hoses/tubing. With the above information we can decipher that a MNPT x MGHT fitting is a connector with male threads on both ends that allows you to connect a pipe with a FNPT end to a garden hose.
Here is an example: http://www.usplastic.com/catalog/item.aspx?itemid=23597&catid=454
But all pipes and hoses are not created equal. To start with, they come in different sizes. So, in the above example fitting, there will be a size indication on it. ¾” MNPT x MGHT connects a ¾” female pipe fitting to a garden hose, which is a standard size.
1” FNPT x ¾” MNPT is a fitting that connects a male threaded pipe to a female threaded pipe. If the pipes in question were the same size, we would not need this fitting because we could just screw them together. This fitting allows you to increase or decrease the diameter of the pipe you are running.
Fitting types:
The diversity of fittings is limited only by your imagination. There’s a fitting for everything. The terms below are useful search terms and are words you might use when asking someone for help at the home center or plumbing supply store.
-90 or Elbow: a fitting with an L shape that joins pipes at a 90 degree angle
-45: an elbow fitting that joins pipes at a 45 degree angle
-Coupler: joins two pipes together without reducing pipe diameter (pipe inserts into fitting), usually slip
-Cap: seals pipe without reducing pipe diameter, usually slip
-Bushing: joins two pipes together and reduces interior pipe diameter (fitting inserts into pipe), usually threaded
-Plug: seals pipe while also reducing interior pipe diameter, usually threaded
-Y or Wye: Joins two pipes running at different angles together into a single pipe, usually slip
-T or Tee: Joins three pipes together in a single plane, slip or threaded are common
-Union: a coupler that can be easily decoupled after final installation
-Barbed fitting: used to connect tubing to pipe or other tubing (see below)
-Ball Valve: used to adjust flow through a pipe, good for coarse flow adjustment
-Gate Valve: used to adjust flow through a pipe, good for fine flow adjustment
-Distribution Manifold: used to split flow from one or two large pipes/hoses to a number of smaller pipes/hoses, usually constructed from a number of smaller pipes/fittings, often with some flow regulator(s) installed.
-Bulkhead: used to connect pipes through the wall of a container. Seal against container (fish tank or sump) is gained with straight threads and a washer. The connections of pipes/hose to the bulkhead can be either slip (pipe), threaded (fitting), or barbed (hose).
Picking pipe and hose:
PVC and CPVC are rigid plastics used in plumbing. They are often used as drains, but PVC/CPVC can be pressurized as well. PVC is better than CPVC for most applications relative to a fish tank. CPVC is more flexible and stays fish-safe at higher temperatures, but the temperatures we’re talking about here are lethal to most fish, so PVC should be fine. PVC pipes and fittings vary by the thickness of the plastic. In your local hardware store you are likely to encounter SCH 40 and SCH 20; perhaps SCH 80. Most hobbyists use SCH 40, the rationale being that SCH 20 has a higher risk of failure when pressurized and SCH 80 is overkill unless you are using a very high pressure in your lines. This is an important consideration because the prices vary between thicknesses with 20 typically being cheapest and 80 being more expensive. Online you can find many other thicknesses/colors of pipe.
Flexible vinyl tubing is readily available at the home center and is fish-safe. Hose/tubing are sized in two dimensions, ID and OD. The difference between ID and OD is the thickness of the tubing wall. Match up tubing ID to barb ID to fit connections together. There are lots of other tubing types available online with varying colors and levels of purity etc. Silicone tubing can be handy in some applications because it is soft and pliable. These same traits make silicone tubing something to avoid in a pressurized line. Distribution manifolds can be easily constructed from tubing using a hole-punch and barbed fittings that you press into the hole.
PEX (Cross-linked Polyethylene) is another material to consider. This is a semi-rigid plastic tubing that bridges the gap between flexible tubing and ridgid PVC pipe for return lines. It can be pressurized to normal household pressure and can be plumbed very easily with either a clamping tool or slip on fittings. PEX is probably not a good choice for a drain line. There are many different sizes of semi-rigid plastic tubing available online and might be useful as pressurized return lines. This is an area that I have not fully explored yet, so maybe someone else will jump in with more details/advice about this material.
Plumbing:
Once you have your parts put together, dry-fit it all together to be sure it looks/works the way you want it to. Getting a water tight seal is the next step.
GHT connections: Straight threads seal with compression against a rubber washer. Inspect the washer for defects before installing. Do not over-tighten. Tight enough that it doesn’t leak is what you are going for.
NPT connections: Tapered threads seal with compression against the threads. To be water tight, you need to use Teflon tape between the male and female components of the threads.
NPTF connections: These tapered threads seal with compression against the threads. A water tight seal is formed with Teflon tape via the crushing of the threads against each other. These are usually metal fittings that need to be mechanically tightened in order to be water tight whereas NPT connections can often be made water tight with hand-tightening.
Slip connections: Watch youtube videos on how to “glue” PVC. It’s easy to do this correctly, but you do have to be quick. It is not easy to be quick and not make a mess. In a nutshell, the process is four stages and permanently binds pvc parts: 1) clean overlapping parts with solvent, 2) coat overlapping parts with primer, 3) coat overlapping parts with cement, 4) combine pieces together using a twisting motion to acheive the final plumbing configuration. There is debate about the necessity of the primer step among professional plumbers, but primer IS required for some cement types, so be sure to consider this when you buy the materials and follow the manufacturer’s directions. Once you are done gluing, give time to cure before using. Incorporate unions into your design so that you can more easily go back and fix mistakes or alter your plumbing in the future.
Barb connections: attach hose to barbs using a stainless steel hose clamp. Do not over tighten. The hose will go onto the barb easier if you wet the interior of the hose or outside of the barb.
This information is meant to apply to plumbing for aquarium water, not air (some differences exist).
Codes:
All of the fittings and pipes you are going to be playing with are labeled with codes that tell you what they are.
-NPT: National Pipe Thread (Tapered)
-NPTF: National Pipe Thread Fuel (Tapered)
-GHT: Garden Hose Thread (Not tapered)
-Spigot or M: Fitting with threads on the outside of the fitting
-Socket or F: Fitting with threads on the inside of the fitting
-Slip: Fitting with no threads
-PVC: Polyvinyl Chloride plastic pipe
-CPVC: Chlorinated Polyvinyl Chloride plastic pipe
-SCH ##: thickness of the pipe wall
-ID: Inner Diameter
-OD: Outer Diameter
Fittings:
A fitting is used to connect lines. Those lines might be pipes or hoses/tubing. With the above information we can decipher that a MNPT x MGHT fitting is a connector with male threads on both ends that allows you to connect a pipe with a FNPT end to a garden hose.
Here is an example: http://www.usplastic.com/catalog/item.aspx?itemid=23597&catid=454
But all pipes and hoses are not created equal. To start with, they come in different sizes. So, in the above example fitting, there will be a size indication on it. ¾” MNPT x MGHT connects a ¾” female pipe fitting to a garden hose, which is a standard size.
1” FNPT x ¾” MNPT is a fitting that connects a male threaded pipe to a female threaded pipe. If the pipes in question were the same size, we would not need this fitting because we could just screw them together. This fitting allows you to increase or decrease the diameter of the pipe you are running.
Fitting types:
The diversity of fittings is limited only by your imagination. There’s a fitting for everything. The terms below are useful search terms and are words you might use when asking someone for help at the home center or plumbing supply store.
-90 or Elbow: a fitting with an L shape that joins pipes at a 90 degree angle
-45: an elbow fitting that joins pipes at a 45 degree angle
-Coupler: joins two pipes together without reducing pipe diameter (pipe inserts into fitting), usually slip
-Cap: seals pipe without reducing pipe diameter, usually slip
-Bushing: joins two pipes together and reduces interior pipe diameter (fitting inserts into pipe), usually threaded
-Plug: seals pipe while also reducing interior pipe diameter, usually threaded
-Y or Wye: Joins two pipes running at different angles together into a single pipe, usually slip
-T or Tee: Joins three pipes together in a single plane, slip or threaded are common
-Union: a coupler that can be easily decoupled after final installation
-Barbed fitting: used to connect tubing to pipe or other tubing (see below)
-Ball Valve: used to adjust flow through a pipe, good for coarse flow adjustment
-Gate Valve: used to adjust flow through a pipe, good for fine flow adjustment
-Distribution Manifold: used to split flow from one or two large pipes/hoses to a number of smaller pipes/hoses, usually constructed from a number of smaller pipes/fittings, often with some flow regulator(s) installed.
-Bulkhead: used to connect pipes through the wall of a container. Seal against container (fish tank or sump) is gained with straight threads and a washer. The connections of pipes/hose to the bulkhead can be either slip (pipe), threaded (fitting), or barbed (hose).
Picking pipe and hose:
PVC and CPVC are rigid plastics used in plumbing. They are often used as drains, but PVC/CPVC can be pressurized as well. PVC is better than CPVC for most applications relative to a fish tank. CPVC is more flexible and stays fish-safe at higher temperatures, but the temperatures we’re talking about here are lethal to most fish, so PVC should be fine. PVC pipes and fittings vary by the thickness of the plastic. In your local hardware store you are likely to encounter SCH 40 and SCH 20; perhaps SCH 80. Most hobbyists use SCH 40, the rationale being that SCH 20 has a higher risk of failure when pressurized and SCH 80 is overkill unless you are using a very high pressure in your lines. This is an important consideration because the prices vary between thicknesses with 20 typically being cheapest and 80 being more expensive. Online you can find many other thicknesses/colors of pipe.
Flexible vinyl tubing is readily available at the home center and is fish-safe. Hose/tubing are sized in two dimensions, ID and OD. The difference between ID and OD is the thickness of the tubing wall. Match up tubing ID to barb ID to fit connections together. There are lots of other tubing types available online with varying colors and levels of purity etc. Silicone tubing can be handy in some applications because it is soft and pliable. These same traits make silicone tubing something to avoid in a pressurized line. Distribution manifolds can be easily constructed from tubing using a hole-punch and barbed fittings that you press into the hole.
PEX (Cross-linked Polyethylene) is another material to consider. This is a semi-rigid plastic tubing that bridges the gap between flexible tubing and ridgid PVC pipe for return lines. It can be pressurized to normal household pressure and can be plumbed very easily with either a clamping tool or slip on fittings. PEX is probably not a good choice for a drain line. There are many different sizes of semi-rigid plastic tubing available online and might be useful as pressurized return lines. This is an area that I have not fully explored yet, so maybe someone else will jump in with more details/advice about this material.
Plumbing:
Once you have your parts put together, dry-fit it all together to be sure it looks/works the way you want it to. Getting a water tight seal is the next step.
GHT connections: Straight threads seal with compression against a rubber washer. Inspect the washer for defects before installing. Do not over-tighten. Tight enough that it doesn’t leak is what you are going for.
NPT connections: Tapered threads seal with compression against the threads. To be water tight, you need to use Teflon tape between the male and female components of the threads.
NPTF connections: These tapered threads seal with compression against the threads. A water tight seal is formed with Teflon tape via the crushing of the threads against each other. These are usually metal fittings that need to be mechanically tightened in order to be water tight whereas NPT connections can often be made water tight with hand-tightening.
Slip connections: Watch youtube videos on how to “glue” PVC. It’s easy to do this correctly, but you do have to be quick. It is not easy to be quick and not make a mess. In a nutshell, the process is four stages and permanently binds pvc parts: 1) clean overlapping parts with solvent, 2) coat overlapping parts with primer, 3) coat overlapping parts with cement, 4) combine pieces together using a twisting motion to acheive the final plumbing configuration. There is debate about the necessity of the primer step among professional plumbers, but primer IS required for some cement types, so be sure to consider this when you buy the materials and follow the manufacturer’s directions. Once you are done gluing, give time to cure before using. Incorporate unions into your design so that you can more easily go back and fix mistakes or alter your plumbing in the future.
Barb connections: attach hose to barbs using a stainless steel hose clamp. Do not over tighten. The hose will go onto the barb easier if you wet the interior of the hose or outside of the barb.
