Just got back from finishing a pond this afternoon. Only 15 x 30 with a 1hp pump. The hard part...12 tons of rock. My hopes I will have a back rub when I get home tonight!
Medowlark is right...line the thing. Be sure to use at least a 45 mil EPDM rubber liner and install an underliner prior. The underliner will allow for protection of roots and rocks as well as allow for gases that tend to build up under liner to escape.
With depths of 2-5 foot deep, you are sure to get an algae problem in such a small pond. Working at Sea World of Florida, we had a saying when working with the large aquariums, "Dilution is the solution". It is easier to get away with out filtration with larger water bodies. In a small one such as your plan, some sort of filtration will most definately be needed.
In my past 20 years of working with water, I have used and tried to invent my own filters for ponds. I have been happiest with the product line we carry now. It is simple to install and very reliable.
Using the 25 x 50 and average depth of 3.5 feet, that comes to 4375 cubic foot. Multiply that by 2.2 (gallons in a cubic foot) and we get 9625 gallons for your pond.
A liner with a grande biofalls filter and a signature series skimmer that has a filter and pump inside will be sufficient enough to filter that size of a pond. Ours here at the shop is 7,000 gallons and we have no problems keeping it clear. There are no shade trees so sun is beating down on it all day.
The key is to line the entire pond with rocks, there is a formula for that too so it looks more natural, making sure you leave areas for plant pockets and plant up to 40% of the pond with aquatics. Finish it off with either KOI and goldfish or game fish, but not both for you will be feeding the game fish a very expensive dinner.
Here are photos of ours today. As you can see, no problems with algae. Since this is my sign, I do have a floating fountain and water garden diffuser in the pond too.
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