If you are just starting out with ponds and especially if you are doing small ponds, then perhaps I can help you bypass a lot of guesswork and get what you need right from the get-go.
First off, I live in the Sonoran desert in southern Arizona. It simply means during the summer, the temperatures in my ponds go up substantially, especially my pond out front of my house that can hit 95 or even high degree water temperature.
Amazing how Koi and Goldfish can withstand such temperatures. I do throw in ice during the summer, but it's not like it's a cool swim for those fish in there after I throw it in there, just brings the overall temperature of the pond down a couple of degrees.
Well, let's get started. First off, the filtration system. If you are going to have fish in your pond, you are going to need filtration. I spent quite a while reading all kinds of information about this particular subject before I just threw my hands up in the air and started looking for a waterfall filter. Basically, it's a plastic container maybe a foot and a half tall and a foot or so wide. It has an inlet at the bottom for water to be pumped into it and then the water "falls" off a large lip at the top of it. I have 2 of them. They both have bio balls in them - you will need those - and they both have the thick filter media. I also buy much thinner filter media on eBay to help further filter the water.
This and the pump that supplies the water to the filter are the most important parts of your pond. Without them, your fish are probably not going to do very well. BTW, you can be hard-plastic liner pre-formed ponds or you can buy a liner and dig a hole in the ground. I have the preformed liner WITH a liner in it. I also turned a 400 gallon galvanized metal horse trough into a pond as well.
The waterfall filters are the best, IMO, but there is also a much smaller, compact filtering system you can buy on eBay or wherever that will also do the trick. The point of the waterfall filters is that the water is streaming off the top of the filter into your pond and yes, you get the nice sound of water falling into a pond with them.
The pump is the other half of the equation. Smaller is not better, especially with fish. More Gallons Per Hour (GPH) is always better. In all 3 of my ponds, I have pumps that are sized large enough to cycle the entire contents of the ponds twice per hour. That's my equation after fooling with this for quite a while. So, if you have a 400 gallon pond, I will buy an 800 gallon per hour pump for it. Sounds excessive, right? Not at all. It is, as far as I'm concerned, the perfect sized pump for the amount of water in your pond. You can go ahead and buy, say a 150 gph pump for a 400 gallon pond and you are going to have a small trickle of water. If that's what you want, then go for it, but it probably won't work with your filtration system too well and if you want that pleasant sound of falling water, you are not going to get much of that with such a small pump. I WENT that route. I started out at 75gph, then 125, 150, 200, 350, all the way up until I got to 800gph and then? I was definitely happy with the results.
That was the hardest part of this setup that I had to deal with. How big of a pump. Well I just give you some good advice. A larger pump will pick up all the junk that floats to the bottom of the pond as well. There IS no buildup down there to have to fool with. If you have enough fish, they stir up the stuff on the bottom and that pump sitting down there sucks the junk - fish poop to be precise- right out of the water.
Next? UV sterilizer. Not all ponds need them. I have one pond that doesn't have an algae problem, my other 2? The water turned pea green. You couldn't see an inch down into the water. I bought the smallest UV sterilizer I could find and they both work great! I don't WANT green ponds, is the point. I want to be able to see my fishies in there.
You can get new pumps and UV sterilizers at great prices on eBay. That and sometimes Craigslist. I bought both my waterfall filters from 2 different people who were shutting down their ponds and were practically giving the filters away. I will never buy a used pump, though, all of them I have bought new on eBay and haven't had a problem, same with the UV sterilizer.
Fish? Took me quite a long time. I wanted Koi. I acquired a lot of pond sized goldfish for free, the Koi were a bit more elusive. But, I kept going to Craigslist, day after day after day, looking for the deals on Koi, all of them were deals made from individuals that were shutting their ponds down permanently. They wanted to get rid of them but wanted them to go to good homes. Your small pond can accommodate a large number of fish. The more fish you have, the more fish food you are going to go through and you will also find you will have to clean out that filter more frequently.
Fish food. I buy large bottles of Goldfish flakes at Walmart, I buy 20 pounds of Koi pellets at a time off of a supplier on eBay.
Additives. Well, if you have just filled your pond up with fresh city tap water and then are going to put fish in right away, I would suggest you get some liquid that is sold at Walmart amongst other places that helps remove chlorine from the water and also reduces stress in fish. I mention Walmart cause' I did my ponds on a budget. I didn't skimp, but I did buy some used stuff.
Plants. You don't need a green thumb to grow water plants and they do amazingly well with out ever doing anything to them. My Taros in one pond have grown incredibly large (elephant ear plant). My Umbrella plants are growing in all 3 ponds. I have Yerba Mensa in one pond that grew from a tiny plant into a behemoth!! Water lillies are also good for keeping your pond a bit cooler if you have direct sunlight hitting it and the water is heating up in summer. I have all of those and also parrot feather in one pond. With my ponds, it was hit and miss. Some plants grow better under shade, others grow better in direct sunlight. I got all of my plants off of Craigslist locally from people either selling them or giving them away.
When an owner of a large pond decides it's time to clean up the plants, many of them will write up an ad on Craigslist. They almost always give the plants away. 90% of my plants were free, only the water lillies did I have to pay for. You really WANT plants in your pond. Natural beauty and helps keep your pond clear. Actually, I don't have large ponds but I occasionally have to prune back some plants, just posted an ad for freebies today and have had several inquiries.
Once you get your pond or ponds going, they aren't much maintenance. Clean out the filtering system once in a while and sometimes the pump gets clogged up with junk that it has accumulated. Feed the fish and make sure the water level is good in the pond. Fish are fun to watch and it's cool to see your plants grow over time.
Also, one last thing about the plants. Whatever kind of container you have, that's what you can use. Cut a milk jug in half and there you have a pond planter. Put the plant in it, fill it with dirt and cover the dirt with rocks. It's as easy as that. This is not complicated stuff, but there is a bit thrown at you at the very beginning, I do hope this article will help.
Edited: Aeration. I found that my small ponds with lots of fish in them were not sufficiently aerated in the summer months by the act of the waterfall filters alone. I ended up buying small air pumps either off of Craigslist or yes, at Walmart, the same you would use in an aquarium. How did I know that the ponds weren't sufficiently aerated? The fish were up at the surface, gulping at air. It was pretty easy to determine the problem and even easier to address it. A pump with at least 2 outlets, the air tubing and yes, air stones. As soon as I did that, the fish were no longer sitting at the surface.
Water changing. Well there's a lot on the net about it, some say to do it once a month and some say to do it twice per month. They all vary on what percentage of water to change out. I haven't needed to change out any water in my small ponds. Evaporation makes the water go down enough that I have to refill it periodically with fresh water. I do think it good to have to put in fresh water here and there, I have no scientific reasoning behind it, I just think that the fish do better if they get fresh water infused here and there. Well, my ponds will go down a couple of inches per month in the winter and a lot more in the summer due to evaporation alone. So, that's all I do. Just refill the ponds.
Edited: Aeration. I found that my small ponds with lots of fish in them were not sufficiently aerated in the summer months by the act of the waterfall filters alone. I ended up buying small air pumps either off of Craigslist or yes, at Walmart, the same you would use in an aquarium. How did I know that the ponds weren't sufficiently aerated? The fish were up at the surface, gulping at air. It was pretty easy to determine the problem and even easier to address it. A pump with at least 2 outlets, the air tubing and yes, air stones. As soon as I did that, the fish were no longer sitting at the surface.
Water changing. Well there's a lot on the net about it, some say to do it once a month and some say to do it twice per month. They all vary on what percentage of water to change out. I haven't needed to change out any water in my small ponds. Evaporation makes the water go down enough that I have to refill it periodically with fresh water. I do think it good to have to put in fresh water here and there, I have no scientific reasoning behind it, I just think that the fish do better if they get fresh water infused here and there. Well, my ponds will go down a couple of inches per month in the winter and a lot more in the summer due to evaporation alone. So, that's all I do. Just refill the ponds.