any ideas? I was looking at prices online and about had a heart attack. My mom won't like this, but it's either that or she helps pay, lol.

Now, that is a challenge! If you are handy with building concrete containers, then you can build one big one, about 6 ft by 2-3 ft, and make 3 compartments within that box. In the first one comes nothing, that is the box where the big waste like uneaten food and poop will settle down. All you have to do is vaccuum the floor off every now and then.
In the second one comes anything that will hold smaller particles. That can be either sponge material like it would be for an Aquaclear filter, the #1 sponges, you can get those in bulk in hardware stores. Or fiber brushes or something.
In the 3. part comes your bio material. Anything that has a porous surface will do there, like cut up pieces of a plastic plumbing pipe, or - believe it or not - the cheap hair curlers from the dollar store!

There must be a hole between all those compartments for the water to flow into the next one. Between #'s 1-2 the hole should be in the upper part, so that the bigger particles can settle down on the bottom. YOu have to experiment on where to put the pump that runs the whole thing.

If you don't have anybody doing the concrete boxes for you, try something like rubbermaid feeding throughs. They come in different sizes, from 100 gl to 300 gl, and range in prizes from 100-180$. At least thats what I paid for my 300's. You can either get 3 of the smaller ones and let them run into each other, fed by gravity, sort of like a waterfall. Or you can make compartments in the 300 gl, with the same "ingredients" than the container boxes. Water comes in one end, runs through all 3 compartments, and goes out the other.....

The trick is basically to be a bit inventive on what to use as containers, and it still saves you a ton of money, compared to a bought filter for thousands of $$$$....
Good luck building!!

Now how would you go about hooking up the pump? and could you use the same pump to create a waterfall (like, the water goes through the filter, then gets pumped up to the top of the fall, where gravity takes over?)

Also, could I use some of the larger plastic containers from nooppee? I saw some of those rubbermaids at my local hardware store today and the 150 gallon was almost $200......(I was thinking of creating a small rubbermaid pond, but with the stupid rules my parents have it won't happen).

Thanks!!

The thing with those rubbermaids from nnnnnn is, that they bend out on the longer side of the container when the water is all in. The biggest one I always found was the 55 gl, and when a certain amount of water is filled in, the sides bent outwards.
If you try the setup with a few containers running into each other, then one pump would be positioned in the water, leading water up to the first, and highest container, via a plastic tubing, and from there it runs gravity fed downwards. As for the concrete containers, I have seen those at a few koi keepers houses during pond tours, but never figured out where they exactly had the pump(s).

An option I completely forgot, and one of the filters I actually use, are the fishmate filters. Here is a link on how they look like and work....
Fishmate filters

They also come with a UV sterilizer installed, and also in various sizes....
Fismate/UV combo
I have the latter ones on 2 of my containers installed, and they work really well. The good thing about them is, they are affordable, and you can connect them to each other via tubing as well.

One last thing, but equally important, so I found, is some sort of plant setup to help with the filtration. I did come to enjoy water gardening quite a great deal, and while researching plants, I dicovered that there is a group of plants that is very helpful in absorbing waste. They are called oxygenating plants, and include water hyacinth, water lettuce, parrot feather and duck weed, just to name a few. Those all have a very dense root system, which can eat up a big amount of waste.
And there is nothing more beautiful than a piece of your pond, away from the koi of course , that has pond flowers included. Papyrus plant for height, water hyacinth for their wonderful purple blooms, and just for their beautiful appearance (besides the waste cleaning effect) I have set up a rubbermaid with plants on almost all of my ponds....

AH! I love those prices!!!

Thank you SO much!