I am not a very systematic thinker (if I can put a filter together, anybody can!), so sorry if these directions are kind of garbled. Let me know if you need clarification.
Here is a picture of the set-up and how the filter is incorporated into it. This is a 50gal Rubbermaid stock tank with probably 35 gal water (Oliver is tiny):
And here's the filter itself:
This is a gravity-driven filter. A sump pulls water from the tank and deposits the water in the topmost bucket. Each bucket has holes cut in it and is filled with a different kind of media. Gravity pulls the water from one bucket down to the next one, and eventually the water drains out of the bottommost bucket right back into your tank.
Materials needed
-Sump pump (rated a gph that is about 5x the amount of water in your tank. Mine is a 210gph pump)
-Plastic tubing
-3 2-gal paint buckets and 1 lid (you may need to increase the capacity of your buckets if you buy a faster pump) Make sure that when you stack the buckets, at least 3 inches of space are left between their bottoms.
-Prefilter foam for the pump
-Filter media - polyester filter fabric (looks like quilt batting), biomedia, and carbon
-Two long, narrow nylon bags for carbon (they have these at Petsmart for $0.99)
-Small funnel
-Four signposts (these are for the stand. I don't have a saw or anything so I had to buy ready-cut wood. If you want to buy sturdier wood and cut it yourself, go for it!)
The pump:
Depending on the pump you buy, your prefilter will vary. For mine I cut a piece of the prefilter foam, unscrewed the cover from the sump input, stuck the foam in there, and reassembled. You can see the prefilter in the above picture. This is important to keep debris out of the pump mechanism.
Attach the tubing to the pump output. The tubing should fit without silicone glue or anything.
I put the sump on the opposite side of the tank from the filter to improve water circulation. Tubing directs the water around the outside of the tank to the top of the bucket.
Now for the filter itself:
You'll need something that can cut through the plastic buckets. I didn't have a good drill bit for this so I had to turn on my gas stove, heat up the tip of a steak knife in the flame, and use the hot knife to melt through the plastic. Needless to say I will never use this knife for food again.
The first (top) bucket will disperse the water coming out of the tube and provide some finer debris filtering.
Cut lots of little holes (smaller than a dime) in the bottom of this bucket. Like this:
Cut a piece of filter fabric and place it over the bottom of this bucket.
The second bucket is for your biomedia.
Cut about 4 holes (just small enough that individual pieces of biomedia can't fall through) in the bottom of this bucket. Don't put any holes in the center of the bucket. Sorry I don't have any pictures of those holes now that the filter is set up. But here's a picture of bucket #2 with biomedia and water in it:
Put in however much biomedia you think you need. Make sure the pieces are not plugging any of the holes.
The third (bottom) bucket is for carbon and another piece of filter fabric, and the water will drain from this bucket into the tank.
Measure the circumference of the widest point of your funnel. Make sure you measure the circumference inside the lip of the funnel, not outside!!
Using this measurement, cut a circle in the center of the bottom of the bucket, creating a large round hole. Put the funnel through the hole so that its lip rests right on the bottom inside of the bucket. (Note: cutting a perfect circle is reeeeeeally hard to do. I didn't do it perfectly, but it's OK. If you want, you can seal the funnel to the bucket with silicone caulking, but you'll have to wait a few days before you can use your filter if you do this.)
Because I didn't seal mine, some of the water seeps out along the outside of the funnel like this:
Cut a piece of filter fabric to fit the bucket and lay it flat along the bottom. This will keep the carbon bags from accidentally rearranging themselves and falling into the funnel, which could back up the system.
Fill your two nylon bags with as much carbon as you think is necessary. Close the bags tightly and arrange them in a circle on top of the fabric, along the circumference of the funnel, but not directly over it. Sorry I don't have a picture of this. I've disassembled and reassembled this filter so many times I don't want to do it again just for the picture!
Lid
Cut a hole in the center of the lid so that the tubing fits through it.
Test each bucket one at a time.
Fill the stock tank (or a test storage bin set up in your bathtub) to the water level you plan to keep your tank. Matching water levels is important to get an accurate test because the height of the water over the pump affects the pump's speed and output.
Arrange the sign posts over the tank like so:
Stack the first and second buckets. Put the lid on the top bucket.
(Testing the first and second buckets will get your signposts wet. That's OK.)
Place the pump in the water and put the other end of the tube through the hole in the bucket lid. Push the tube down so there is only an inch of space between it and the bottom of the bucket. Plug in the pump to get it started. Let the water flow for a half hour (but keep a close eye on it the whole time). The water should flow very quickly out of the first bucket. If the water level rises past 1", you should cut more holes in the bucket.
Gently lift the first bucket to see what that water level inside the second bucket is. If it's above 2 inches, or if there is any water backed up into the first bucket, you'll need to cut some more holes in the second bucket. If the water drains from the buckets too slowly, it will back up and overflow.
HOWEVER, you want to make sure that the water in the second bucket is deep enough to keep the biomedia immersed at all times. This is important for bacteria growth. Cutting these holes is a careful balance between the water moving too slow and too fast. Start with too FEW holes, because you can always make more holes.
Test the third bucket the same way you tested the second, with the first bucket on top of it to disperse the water.
If everything seems to be fine, stack all of the buckets on top of each other in order and do a complete test. Let the water run for at least an hour to make sure there are no back ups. Lift each bucket while the filter is running and peek inside. If the water level in each bucket is below 2" you're in business!
Set it up over your stock tank and go!
Some points of interest:
1. I don't recommend doing this over a glass tank. I would worry about the glass holding up the weight of the water in the buckets. This filter isn't too heavy, but it isn't light either, once there's water in it.
2. Not sealing the funnel to the bucket has been ultimately a good thing. The water that sneaks out and slides along the outside of the funnel actually does a great job of aerating the water and I don't have to use an airpump. But depending on how good of a cutting job you did, you might have water dripping loudly all over the place, in which case you might want to go ahead and seal it.
3. I use Seachem Matrix natural biomedia. I love it. Oliver's tank is cycling along quite nicely.
4. The closer the funnel is to the water line in your tank, the quieter it will be.
5. Any questions?? Things that need clarifying?
I hope this is helpful to people hoping to make a DIY filter. This is a great option if you're like me and are terrified of leaks. If this filter leaks or overflows (which it did on my first try), it leaks right back into the tank, so you don't have to worry about coming home and finding the entire contents of your tank flooding your house. Also, it was very very cheap. Less than $100!
