I've been thinking about your post and truthfully, all I can suggest is for you to look at the Skippy site and see if you understand it. I've been trying for 45 years to understand why my DH is incapable of understanding what to me are simple instructions and I don't have any idea how unmechanical you are. What you want to do is certainly do-able. One of our currently missing ponders did just that but she went with a veggie filter and settlement tanks. If I was doing it I might add the veggie filter (as a bog) in addition to the Skippy but that is a personal preference. I'll let someone with a bigger pond than mine discuss the pump and plumbing with you.
The green water you are experiencing is algae. For a pond the size of yours I can't imagine going without a UV light. especially if the pond/pool is in the sun. If you can get the algae under control by any means, a decent sized UV light will maintain and control new blooms. I know you want a natural pond but you have to remember that what you have is not natural. It is a closed system and you will need to intervene in place of Mother Nature who would turn your pond into a smelly mess otherwise. Covering the surface of the pond with plants to create shade will go a long way to clear water. The trees and such you mentioned sound like a good starting point for casting shade and those new plants sound like you are going the right direction. Now, I expect the experts are going to be anxious to give their advice. Sandy

If I were you, I would have more than one filtration system.
I have a 15,000 gallon pond and have several filtration systems.
Each system is on its own 20 amp electrical circuit so if one system trips or has a problem, I still have other filtration systems running.
I have a true water garden with plants, comets, shubunkins, and Golden Orfe.
Since I have no koi, the pumps turn the water over every 1.5 to 2 hours.
If you add koi to your pond, make sure that the pumps turn all 9000 gallons every hour.
I used 55 gallon barrels for one of my filtration systems.
I use matala matting for the filter material. From the barrels it leaves to a Wlim external pump with a 4200 gph to a 300 gallon veggie filter. and then a waterfall back to the pond.
I have two bottom drains.
My second system has a skimmer and a tsurumi pump at 4800 gph. it filters through matala matting to a biofalls.
I am in the process of adding another skimmer and another bottom drain.
I have several other water features with some smaller pumps averaging 2500 gph.

The barrel filters were fairly easy to make with the matala filter material being the most expensive.
The 300 gallon stock tank was on sale as it had a small crack at the very top that did not affect its stability. I used a jigsaw to cut out a hole for the weir of the waterfall. If you google barrel filters and veggie filters you can find lots of instructions.

I do not have a UV. I have a lot of plants and except for a small algae bloom in the spring, my water is clear.
You may find a UV works for you.

I hope this helps.

Sleepless, I think I pretty much understand the skippy site. Do you think my pond needs a 300 gal rubbermaid tank, and if so, what kind of pump? I hadn't thought of a UV light. Wonder how much they cost in electricity to run. I do understand that I have to do something to prevent the "natural" destruction of the pond by not doing anything to filter.

LSST, I don't quite understand why you need all of your filters, and are adding more. I am sure it is beautiful, but I want simplicity. I have three kids and am kind of klutzy with mechanical things.

Thank you both very much. If I get started with the skippy, how do I know what size to order, of stocktank and pump? Thanks.

The reason I have several systems is to keep the water aerated and to prevent areas of the pond from becoming stagnant. I find I still need more filtering as my pond is creating more gunk than my current system can handle.

I also like the look of more than one water feature.

Maybe someone who has a similar sized pond to you will chime in.

It's been a while since I read the Skippy site so I went back to see what if anything it said. Sure enough, it had suggestions. See below.
Of course a lot depends on the amount of stuff that ends up in the pond and how much filter material you have and how much water is being pumped. Mine often depends on the mood of the local raccoons. No two ponds are ever exactly alike. Remember, this hobby has variables. I have 1,000 gallons in the pond, a 100 gallon Skippy and a 1582 gph pump. I use a separate small pump for a UV light. Sometimes I hook up a floating skimmer which is pretty much a waste of money unless I can figure a way to make it more useful. One of these days I will make the pond bigger because the fish are getting huge. I may just add a couple more Skippys to make a bunch of catch basins for waterfalls. Sandy

"Parts list:
A Rubbermaid stock tank. -- Old Style -- New Style.
They come in sizes 70gals, 100gals, 150gals, and 300gals. (Also available is a 50gal size but we find it too shallow to be used as a filter.) Which one is best for you? Of course the size you will need is dependant on the size of your pond. The bigger the pond the bigger the filter you will need. Makes sense, but, how about a rule of thumb? Let us say this first, these filters really work. We have a few thousand in use at this point and have not had a single failure. That is, every one we have put in is working as expected. Actually, using the S.A.G. approach (Smart As_ Guess) we think the following will work:
Updated as we have found these filters to work better than even we expected...
70 gal size filter at least to a 2000 gallon pond size
100 gal size filter at least to a 3000 gallon pond size
150 gal size filter at least to a 4500 gallon pond size
If you need a larger filtration system, we now suggest that you build two 150 gallon size filters.
Worst case: If you built a filter so small that it did not work for your pond, you could just build another and connect it to the first one and double the capacity of the filtration process. Or, add another in a different area of your pond, doubling the filtering capacity and adding more circulation."

So I guess I should do two 150 gallon filters. It would be nice to aerate two different sides of the pond. Would that mean I'd need two pumps? What strength should the pumps be, I wonder? Which is more expensive to run, 2 smaller pumps, or one bigger one? I'd like to minimize the daily electricity costs.

The pond is only 10x20 so it doesn't seem that big really. The volume is greater because it's so deep.

Thanks so much.

" I want simplicity. I have three kids and am kind of klutzy with mechanical things."

A skippy is the least mechanical type of filter you could have. There are no moving parts and once it's built -it's done. Just attach your pump hose and plug the pump in.

Good luck! Do you have any pics?

I'm told the external pumps are the most efficient, but I haven't really kept track of those. I need to keep my pump out of the reach of the raccoons and they still managed to wreck one. I can think of several things to consider. Cost of the pump, power usage, water volume, reliability. We need some experts here, please!
Two pumps might be ideal if one died. They could be backups for each other, but the cost would be much higher for price, cost to operate, housing. We often suggest keeping a second pump on hand as backup but the cost might be substantial as a dual system. I think most here have gone for the Y arrangement because it is less complicated to take care of one pump.I want an external pump when my pond grows up.
You need a prefilter to keep bigger stuff out of the pump. This is pretty simple and there are many ways to do it. I attached a prefilter for a well in front of the intake tube. It is a cylinder with a lot of narrow slots to pass the water while holding back debris. Something similar should be available in most big box or hardware stores that carry well materials.
I would like pictures, too.
I totally forgot. There are at least two others who have converted from pool to pond. I'll see if I can locate their pictures. Sandy

My favorite type of filter is a mechanical type filter. Take some sort of an old tub (since you have 9000 gallons will have to be large or more than 1.) Using a pump that is in the pond, put an inlet to the bottom of the tub, you might tip it so that it runs out the top or if you want it level it will spill over all sides, fill it with gravel of your choice, and plant some marginal plants in it. After you have your pump up and running it will be beautiful. I have made these on a station inside my pond for it to come over all sides and drain back into the pond, I have made them on the side and it will act like a mini waterfall, however you wish. You can use quilt batting in the bottom to get a good strain at first, then change it out to gravel/rocks as it clears if you want. Being somewhat unmechanical myself, this has been the easiest for me to do. If you don't want to be in the water or deal with a pump in the pond an external will be ok, I have always had submerged pumps though. :)