Didn't get many photos of the main pre-filter, although it was a great inexpensive idea. I simply fitted 2 buckets together - top left of this picture (which are free, as I am always buying chlorine for the pool). Because the upper lip is a couple of inches thick, the buckets fit together with about a 2 inch space between the 2 bottom panels. I drilled many large holes in the bottom of each bottom panel, than lined the space in between with filter material (the blue/white stuff you can buy in a sheet to cut to fit your own filter). I also drilled holes into the cover, and screwed it on, with another piece of the blue filter material. There is a large hole in the center for the hose. Inside of the filter is room for pretty much any size pump. I have a 1000 gallon per hour in it now; but could easily go larger or smaller if I want. Submerged pumps run cooler and last longer, as well as add some heat that is valued during the winter months. This is easily changed, easily cleaned, and has much greater capacity than a prefilter you would buy for about 25 times the price.
After the prefilter, the water is pushed through the 'Jacuzzi' brand cartridge filter (foreground of picture above). I ran across this filter at a yard sale a few years ago for about 8 bucks, knowing I would use it someday. This provides very small particle cleaning, for super clear water. Cleaning is a matter of unscrewing, removing the cartridge, and hosing off. I did have to make some modifications, though, as it turned out the Jacuzzi filter was aimed at providing high flow rate, even if in need of cleaning. It therefore offered a couple of 'bypass' methods, to retain full flow. While this would be wanted in a Jacuzzi, in my application, I needed 100% filtration at all times. Using some glue and gaskets, I sealed up all the bypasses.
The next step, and one of the most important in a pond, is the bio filter. This is a virtually maintance free, 'biological' based filter system. To make a long story short, a bio filter breeds 'good' bacteria, which 'eats' the 'bad' bacteria. To be most effective, bio filters need good water flow, and lots of air. On the negative side, they can be expensive. Again, for this reason, I figured I'd build one. The 'filter box' is a poly bin, about 2x1, that I got at Big Lots for 5 bucks. Bio balls are by far so superior to 'cheapo' alternatives as far as bio media, that I did break down and spend the money for them. What you see here is about $15 worth on the web. Basically, you are looking for a lot of surface area, for the 'good' bacteria to 'hang out', while also being exposed to a lot of air. For the amount of space they take, manufactured 'bio balls' can't be beat.
Since air is such an important factor in growing beneficial bacteria, it is highly advantageous to keep your bio material out of the water, but in the flow. I used an 'eggcrate' ceiling panel, which is cheap, and very easily sized, to keep the bio balls off the bottom of the filter. The eggcrate sits on top of some lengths of PVC, and the balls on top of the egg crate.
I then drilled a hole in the side at the bottom, and screwed in a barbed pipe, with a plastic nut snugged up from behind. Although I thought it might leak, it never did - but I did have another major problem with this design. (it is explained below)
The bin I selected had a nice tall top, which, when inverted, fit nicely inside the bin - providing about an inch and a half deep 'distribution' source. I drilled MANY small holes in this, so that the water would flow all across the entire surface of the bio balls below. This even flow is important to the bio (any balls not receiving flow are not going to grow beneficial bacteria), but is also great at further aeration of the water. In any aquatic environment, aeration is always your friend. It provides a great escape mechanism for bad things like ammonia.
This is the final product, in place inside the 'attic area' of the iguana house. After the Jacuzzi cartridge filter, which is removable with quick release fittings, (and outside the cage for easy access) the water travels through first 3/4" pvc, than down to 1/2" pvc, though the 2x6 on the bottom of the cage, and up to the top of the iguana house. I made a couple of design 'errors' that had to be taken care of. First, that barbed 3/4" fitting I provided for an exit, did not provide NEARLY enough flow rate to empty the tank at the rate the water was being fed. As I mentioned, I had a 1000 gph pump feeding this thing, and you can bet that the water moves pretty quickly through the 3/4" pvc feed pipe. Well, without being forced, it wasn't about to come out the little exit barb that quickly. So, the tank filled up, and took the next easiest way out; over the top. I then drilled 2 more 3/4" holes in the bin on the bottom of the front, fitted pvc fittings to them, and provided 2 more exit pipes (the white pvc 90 degree bends on the picture above). The original barb still just has the clear tubing. The thing you see these 3 tubes feeding will be shown later (the water spout); but that is basically the exit source for the filter system.
The other 'miscalculation' I made, was that while sitting on the floor (of the attic), in order for the water to go up and over the 'exit' piece on the left, the water level would have to be so high in the bin, that a lot of the balls were under water. This lessens their effectiveness. I ended up propping up the bin to a level high enough, that the balls would be back above the water line. In the picture you see, the right side is still in the water somewhat, but with some fiddling around, I took care of that (but didn't take a picture)..
Finally, although not yet implemented, is a UV filter. UV is about the best way to keep your pond from turning green. It is also very good at, depending on your flow rate, killing parasites and diseases. I will be putting roughly 500 gph though a 15 watt UV, which is somewhere in the middle of the scale - much more power than is needed to simply kill algea, but not quite enough to kill ALL sorts of disease (but will kill many). (As a comparison, I use a 25 watt UV with a flow of about 600 gph to kill the maximum amount of disease in my largest home aquarium). You should let your pond start cycling before you go with a UV, so for now, my UV is still sitting in its box. Eventually, I will put 'half' the water flow into the bio filter in the roof (which is also where the UV will be housed) through the UV, while the rest of the water will go directly into the bio filter. This way, not all the water going into the bio will be 'sterilized', still providing good food for the beneficial bacteria. Ideally, you always want the UV AFTER every other stage of filtration; but in a pond environment, this is not usually practical. In my case, it would be just like the problem of the one exit tube not being enough to drain the bio filter. Unless you have some way to 'force' the water through, it simply isn't going to go fast enough. In your wet/dry home aquarium filter, the pump is already after all the phases of filtration - since the feeds for the filters are done by gravity. Your tank is on a stand, you pond is not. In the aquarium, you just put the UV after the pump, on the way back up to your tank. In a pond, the pump is typically at the beginning of the cycle, so you have to make some sort of compromise.