The original 3 - 3/4" holes are on the opposite side of  the DIY Carbon Reactor picture.

I added the 3 black spots to show the drain ports for the DIY Carbon Reactor

I personally use my reactors for carbon only. I’ve been told from other hobbyist that they copied and made their own reactor and that they use different media such as some type of Phosphate remover media, Ammonia remover media or a Nitrate remover media.

Parts needed to make my DIY carbon reactor from top down:

1- ½” Male grey barb
1- ½” PVC Female 90 degree elbow
1- ½” PVC pipe x 2.5” long
1- ½” PVC Male 90 degree elbow
1- 3” PVC end cap
1- 3” Test Plate
1- 2” thick sponge
1- 3” PVC pipe x 12” long
1- 1.5” thick sponge
1- 3” Test Plate
1- 3” Coupling / Pipe Connector
1- Plumbers Tape
1- PVC Cleaner
1- PVC Glue

Preparing Parts for Reactor:

Refer to picture web site:
http://www.gettankedaquariums.com/index_file/DIY_Equipment_Main_Page_1.htm

Step1): Apply plumbers tape to threaded end of ½” Male grey barb and attach to threaded end of ½” PVC Female 90 degree elbow.
Step 2):
Take the ½” PVC pipe x 2.5” long, use pipe cleaner and glue and attach to slip ends of  ½” PVC Female 90 degree elbow and ½” PVC Male 90 degree elbow.
Step 3):
3” End Cap must be drilled and then tapped / threaded to except the plumbers taped threads end of ½” PVC male elbow that was assembled above.
Step 4):
Take the 2- 3” Test Plates and use recommend 9/64 drill bit and drill as many holes as possible with out cracking the test plates, the more hole you drill the better the water flow.
Step 5):
To make the sponges: Take a Pre-filter Sponge Block 3” x 4” x 18” used for wet/dry filters. Use one 3” test plate as a cutting guide for on the 4” side of the sponge. You will only get 4 cuts with a little sponge left over. The 3” x 4” sponge block cut in half on the 3” side.
Step 6):
Cut recommended 1- 3” PVC pipe x 12” long or to your desired length.
Take the 3” Coupling / Pipe Connector and drill 3-3/4” horizontal holes for water port exits.

Assembling Reactor:

Step A):
Take the 3” x 12’ PVC pipe and use the plumbers tape and tape one end.
Step B):
On the taped end insert the sponge, and then take the drilled test plate and insert the small side into the 3” x 12” taped end.
Step C):
Take the 3” Coupling / Pipe Connector and use a twist motion in the direction that you applied the plumbers tape till tight, and then put the 3” Coupling / Pipe Connector end on your floor and push down hard until tight.
Step D):
Use a long item to push down the sponge that you placed in so it’s flat against the test plate. Fill with activated carbon to the top and then tap to settle, leave enough room for the other sponge and test plate.
Step E):
Apply 3” End Cap as per Step A), Step B) & Step C) but reverse.
Step F):
Take a 5/8” I.D. x 12” vinyl tubing and attach one end to the grey barb, when attaching the other end to your highly recommended 1200 Maxi-Jet power head. You will have to apply some plumbers tape to the 1200 Maxi-Jet power head to make a tight fit to vinyl tubing connection.

Final Step:

When working on your reactor, remove the 1200 Maxi-Jet power head from vinyl tubing rather from the gray barb end. It’s easier to reconnect to the 1200 Maxi-Jet power head then from the grey barb.
The following is a two fold:
You can use a garden hose w/spray attachment or faucet and use your hand as a coupling to join reactor hose and water source. Turn water on as high as possible, let water run thru reactor until water is clear, drain reactor and repeat one more time.