I've wanted to build this project for some time now. Finally had the time. I esentially made one and a half. The first filter leaked and had some design flaws. so mid way through I changed the whole design.
Disregard the threaded pipe cap and the rounded cap. those were from the original design.

You'll need:
8" of 4" diameter PVC pipe
1 - 4" diameter plug
1 - 4" diameter rubber end cap with hose clamp
2 - female garden hose repair ends
2 - 1/2" threaded valves
4 - 3/4" to 1/2" threaded plastic hose conections
1 - male garden hose threaded (2 pictured for old design)
3 - pvc grates about 1 1/2" tall
1 - 3/4' 90 degree corner tube(has hose ends)
some small hose clamps
PVC cleaner
PVC glue
teflon tape
aquarium silicon
20' - 3/4" tubing
5' - 1" tubing
3 small zip-ties
and of course a pump. mine was a 700gph in line pump

Filter media:
poly sponge(got one for car washing)
knee high stockings( cheap ones in those little plastic balls)
bio-balls

Tools:
Drill
small drill bit
7/8" spade bit
coping saw
utility knife
flat head screw driver
channel lock pliers
cresent wrench
needle nose pliers
tape measure
sanding block
scissors

I may have forgoten a few things and you may need to swap a few parts to fit your needs exactly. I hope I explain it well enough to be understood. Any comments feel free to post. also please rate!

make the pump PULL water in the top and out the bottom. do not plumb it to push water through. it builds to much pressure and WILL blow the plug out of the top. I found that out the hard way. thank God for those valves installed or it would have syphoned my whole tank onto the floor. instead i caught it quickly and only about a gallon spilled.

Step 1: Drilling Holes

Keep in mind the drilling photo was from the old design but the process is the same.the pipe photo is the good design. From the bottom of the pipe just above the rubber end cap drill a hole with a small bit in the center of where your male garden hose adapter will screw in. this will keep your spade bit centered. drill with your spade bit slowly so you don't throw shrapnel around your room.
you may need to smooth it out or slightly enlarge your hole with a bit like the one shown. I didnt make the hole too large. i wanted a good seal. I wrapped teflon tape around the adapter a few times then threaded it into the pipe with the cresent wrench. take your silicon and run a bead around the adapter on both the inside and the outside. i dont suggest smoothing with your finger like i attempted. its difficult to clean off your hands. let this silicon cure for the recomended 48 hours before filling the filter with water for best seal.

the filter baskets will sit just on top of the outlet tube. to keep them laying even i glued a small piece of PVC pipe opposite the outlet tube at the same height.

Step 2: Filter Baskets

These are the grates that cover floor drains. I had to cut them down to just the main diameter so they would fit in the pipe. i used a coping saw. first i made small relief cuts(shown in second photo) as it made it easier when you ran the coping saw around the outside of it. my first one didn't come out quite as nice as i hoped but the other two were better.i just kept testing to see if it fit in the pipe and trimed off an excess with a utility knife. I sanded all the burs off. also i attatched a short zip tie to the basket to make removal easier

Step 3: Valves and Hose Connections

This is the part that you may need to adapt to your situation depending on what your plumbing supply store had. I used the valves to stop the water flow when I need to change the filter media. Put teflon tape on the threaded 3/4" to 1/2" tube conector then thread them each into the valves.youll need to connect your hoses to the pump and to the valves. its very detailed as to which hoses to wear but look at the pictures. also if you should be able to figure out how you need to set it up. I used the female garden hose connectors to attatch to the filter itself. youll need to slide on the hose then use the channel lock pliers to pinch the connector down into the hose. i had to attatch the 90 degree corner to the bottom or outlet of the filter so the hose wouldnt kink. the technique i used to get the hoses to fit was use very hot almost boiling water and let the hose sit in it for 30 seconds or so then take it out and stretch it open with the needle nose pliers. then give it your all and force it onto the connectors. use a towel to help grip and protect your hands. try to get them seated as far on as possible for best fit. if needed because of a slow leak or just for more security attatch hose clamps to the ends also.

also put in hoses to and from the tank. one valve should be on each the inlet and outlet hoses. my hose from the tank happened to be 1" diameter but you'll need to get one to fit your pump.
the hose to the tank was 3/4" just to make things the same.

Step 4: Filter Media

The filter media used was a poly sponge found in the cleaning section of my hardware store. its suggested use was for cars and cleaning because it resisted solvents. it doesnt have the large holes that dish washing sponges have and holds together longer. place your filter basket on the sponge and cut around it with the utility knife. the sponge should be thick enough you can filet it and make two filters from it. make a small hole in the center so the zip tie from the basket can fit through.

the second basket contains the activated carbon. i bought some knee high panty hose from a certain super center and poured a decent amout into it. it isnt stuffed by anymeans. it maybe has a cup worth of carbon in it. loosly tie the end in a knot so you can possibly reuse it. lay it in the basket around the zip tie and rearrange the carbon so it fits nicely.

the third basket would contain the bio-balls. i didnt have any as they were in delivery but the instructions are simple. put them in the basket.

Step 5: Put It All Together

first wash or rinse the whole thing off very well. you wouldnt want to contaminate your tank with anything.
put the rubber plug on the bottom of the pipe. tighten the hose clamp. attatch the tube to the outlet valve. drop in your filter baskets remember bio balls first, then carbon sock, then sponge. push the plug into the top. tighten its screw. screw on the inlet tube. and begin the water flow. hopefully you did everything correctly and it doesnt leak. my first design leaked around the threaded cap thats why i scrapped it.

once its running it should work like a dream. I hope this was explained clearly and detailed. it was my first instructable after all.
thanks for reading!