I just ran across a garden pond calculator. one of mine is 10X15 foot with water 40 inches. That is about 4000 gallons. The calculator says I can keep up to 15 five inch fish. I agree with that.
BUT - I have about 30 large, fat gold fish bigger than 5 inches. I have not bought one since the 90's but they multiply. I have given many away, ever die. About 5 years ago I got 6 small Koi. Now they are big. Four of them are about the size of a German U-Boat. They are huge , I would guess in the 3-4 pound range.
Throw in a few turtles, a couple Bull Frogs and my POND should be on "death watch".
It is not, but it is over stocked.
The pond is crystal clear and all the critters are thriving (and reproducing). Their main diet is the big bags of food from Tractor Supply people buy for CatFish farms. It is cheap, but works really well in my yard. I throw in lettuce when it is in season here in the yard. The Koi and turtles love their greens.
It is about time for me to re-install their bug light. I have a big UV zapper I mount over the end of the pond in Summer. It gives me a light on the pond, and zaps the bugs. I took off the bottom and the bugs fall into the water. At night, the fish, turtles and frogs mob under the light. There are "a lot" of bugs here, and I actually believe this light would be enough to feed them all. The frogs grab some before they are zapped. It is interesting and apparently nutritious for all as all are fat and healthy.
Still, I have a critter overload as to the calculator, and I do agree. I think my pond remains healthy because of my YardCrap filter system. You can buy filters, but they are too small, or too expensive to maintain a 4000 gal pond. Having been in the aquatic critter keeping hobby longer than most of the so called experts have been alive, built my own. I always have, just made this one bigger. The one that runs my Aquarium is home made and overkill. There is no need to pay big $$ for something so simple.
You need aeration, and a nitrogen cycle. The nitrogen cycle breaks down the critter crap into harmless dirt/crap. In Nature, the plants eat the dirt/crap.
Instead of the bottom drain all the "experts" claim you must have, I put in a spillway made with a 6 inch PVC pipe. A bottom drain sucks your small critters off. If you put a screen over it, it clogs constantly. I don't use them, but guess they would be OK if you don't keep small critters. At any rate, they are far from "must have". My "stand pipe" sucks water off the top of the pond. That keeps the surface water clean and in case of a pump problem, the lowest my water can go is full pond. If we get 12 inches of rain in an hour, the highest my pond can go is full pond. That sucking water off the top makes a pond look good. There is no need to pay big bucks for a skimmer if you built it in with a $5 piece of PVC.
The 'stand pipe' accepts water from the surface and it flows to a 100 gallon tub I dug in behind the pond out of sight. As the water falls into the 'stand pipe' and then into the tub, it is aerated. (very important). In the tub are two pumps. One is inside a plastic container used to put out recycled crap on the curb. (I recycled the container). I put the pump in, and packed it with plastic bio balls. These things work well if you use enough of them. They can be found at a decent price on ebay. (you want the large one s for out doors) They also keep the pump from clogging. "EXPERTS" make fun of them to sell you what they offer.
This pump pushes water from the tub to the bottom front of the pond with a cheap PVC line I put in. The water rushes out along the bottom of the pond to give circulation and not allow dead spots. (this is important) This pump runs all year, 24/7. To the eye it is invisible. This pump is not to impress with waterfalls and crap - it does the work and is not for visual effect. It is much more effective for a "critter keeper" than a bottom drain. It will never drain the pond and kill your critters. Simple physics mean a lot in working with water. You can make it work with you, or fight it. Fighting it costs more and does not work as well.
Also in my tub is a second pump. This one pumps water to a second 100 gallon tub I have mounted higher than the pond. That tub is filled with bio balls. (laugh if you choose, but I have years of experience to show they work if you buy enough) This pump is cut off in Winter when the temp drops to 50 degrees as the nitrogen cycle is useless then. Also, no need to feed anybody much then. The turtles don't eat at all. The water flows with gravity through what I call my "Spring House" to "water fall" into the pond. All this adds to Summer aeration. Oxygen runs the nitrogen cycle and the critters need it in the water. This system keeps the pond at O2 MAX.
The result is years of enjoying a clean, healthy pond. This filter system is over kill, and apparently can support a lot of critters. Besides working, it cost a hell of a lot less than what "experts" have to sell you. You might not need this same system if you have less Than 4000 gallons or don't have Koi that are too big for the Eagles around here to carry off. In my 220 gal Frog pond, just need a small pump and a simple box to keep it from clogging. Every Pond is different. The needs for every pond never change and there are NO miracle products. If you just remember one thing from this - "There are no miracle products"
There is always more as you fine tune anything. Many of my audiophile friends swear by funny feet or a plant positioned just right.
I fine tune.
My pond is filled with water plants. Those guys eat the critter crap. Water plants grow like crazy and you never have to water them. My pond is shaded. I have cut back tree limbs as much as I can to keep some Sun coming in for the Turtles, but it is shaded most of the day. Also, between the Summer Pump and the tub I have a UV light hooked in to kill things like paracites and free floating algea. I plug it in about 4 months a year.
Maintenance amounts to cleaning the bio balls. I do it before Spring. I take a wheel barrow load and hit them with the high pressure hose. It takes about an hour a year.
If you ever decide to put in a pond of ANY size, plan a little more, and spend a little more effort and money at that point. If you don't, you will waste your money and end up with the cess pool your friends have. All ponds are different because of needs and environment. Contact me with your plans and I will give you my thoughts. (you could also contact the other "experts" here I guess :wink-new:)
My garden ponds entertain me to no end - kinda like my Beagle. I recommend acquiring both. My smallest pond was just re-activated. It is a big 3 gallon pot my Bride gave me a few years back. It is on the deck with a water plant. In a couple weeks, I will put up a photo and many will be amazed. Water plants grow like crazy and upkeep is near zero.