I've found this short document to be very helpful for ponding. It answers a lot of questions and dispels a lot of myths that circulate in the world of ponding.
Good luck with your pond

Everything in the pond affects water quality. This includes filtration, algae, location, circulation, and the water that fills the pond. Plants are the key to good water quality.
Tap water contains either chlorine or chloramines added by municipal water departments to disinfect water. All water departments will eventually use chloramines rather than chlorine because chlorine forms extremely toxic compounds with many organic substances. These compounds are the most carcinogenic compounds that we are exposed to in our environment. However, chloramines are more harmful for fish. Chlorine is easy to get rid of but does not disappear immediately. It takes 12 to 72 hours for chlorine to naturally disappear. Spraying water or adding air bubbles with an air-stone does not make chlorine disappear faster. Chloramines are a combination of chlorine and ammonia. Chloramines do not disappear for several weeks. Chlorine dissolves gill tissue. A little bit of chlorine will produce scars and a lot will kill. Chloramines are absorbed into the fish’s blood where it binds with red blood cells. The cells cannot absorb oxygen and fish suffocate. Call your local water plant to find out which chemical is used. Water departments are required to supply residents with a printed water quality report that is prepared every 4-6 months. Water must be treated with a conditioner to remove these chemicals and protect the fish. Or a whole house water filter should be attached to your hose when filling or topping off your pond. Fish grow until the day they die. They repair slight damage to their gills caused by caustic chemicals (ex. topping up the pond) although the new tissue may be slightly deformed. Fish (especially Koi) will die prematurely after repeated exposure to chlorine/chloramines. These deaths often occur during the summer when there is less oxygen in water or when they are attacked by parasites.

Tap water also contains metals. Metals build up slowly as more water is added and become heavy metals. Copper, lead, even very high levels of iron can affect both fish and plants. Most water conditioners contain EDTA to chelate and remove metals. Trace amounts of metals are beneficial but too much is toxic. All plants need copper to survive. Fertilizers even contain trace amounts of copper. However, too much copper kills plants. The easiest way to solve heavy metal problems is to do partial water changes.
Ammonia nitrite is found in well water. Tap water contains ammonia nitrate. The nitrate level should be below 40 ppm. If it is higher than 40, there will be an algae problem. Nitrates build up in ponds and plants will eventually remove them. If there are a lot of fish and heavy feeding, additional nitrates are added causing nutritional uptake problems for fish.

In a well-planted pond, nitrates will be close to zero. Ammonia is secreted by fish. Ammonia cannot be seen, tasted, or smelled. It is the preferred nitrogen source for plant growth. Fish are the primary source of ammonia in a pond. Fecal matter is broken down into ammonia but 80 - 90% comes from the gills. Ammonia is extremely toxic to fish but not to plants. Bacteria break down ammonia to nitrite that is even more toxic. Nitrite binds to red blood cells (like chloramine) forming methemoglobin and fish suffocate. It is easy to determine the cause when a fish dies from nitrite poisoning because gills and blood are brown. This condition is called "brown blood disease". Plants are very healthy, water is clear, but fish get sick and die if there is an excess of ammonia or nitrite. The only way to prevent this before it becomes critical is to check for nitrite. Nitrite and ammonia should always be zero. This indicates the nitrifying bacteria and biological filter are working well and the water quality is great.
Another part of water quality is pH. A scale of 0 - 14 represents pH (measurement of acidity). Neutral is 7.0, below 7 is acidic, and above 7 is alkaline or basic (not the same as alkalinity). The ideal pH is from 6.8 - 7.8. If the pH is slightly outside the ideal range, fish color may be duller and plants may not grow quite as well. If below 6.0 or into the 5’s, fish will suffer from acidosis. They will swim erratically, have gill damage, open sores, and try to jump out of the water. If the pH is above 8.5, fish just begin to die. At a pH above 9.0, fish will have alkalosis and organs will fail. With a high pH, fish do not demonstrate symptoms before dying. Carbon dioxide (CO2) produced by fish and plants affects pH. The level fluctuates throughout the day. Plants utilize CO2 during the day and produce oxygen. During the night, plants use oxygen and produce CO2 (photosynthesis). The more CO2 there is in water, the lower the pH. The lowest pH is at sunrise and the highest is about sunset. Test pH at the same time of day and be aware of the cycle when analyzing test results. The pH may also change between measurements made directly from the tap and water that has had time to equalize with the atmosphere. If water is allowed to age a day or two and become mixed with air, the result may change enough to eliminate use of pH adjusting chemicals. Sometimes the result is lower but it usually rises as oxygen concentration increases and CO2 trapped in water pipes disperses into the air. The only way to know the exact pH is to test. The quality of tap water may also vary throughout the year as the source or treatment changes. If there are large swings between the high and low pH during the daily cycle, the KH may need to be adjusted. Carbon hardness or alkalinity is KH (measurement of the buffering capacity of water). If the KH is below 20 ppm, there will be large swings of pH. Ideal KH is between 50 - 200 ppm or mg/L. Tap water is about 50 ppm KH in this area. This is borderline low and water will loose KH as it ages. To increase KH, add sodium bicarbonate or other carbonate. The only way to know exact levels is to test with a simple titration kit.
Tap water is does not contain salt. Even though water garden fish are not saltwater fish, they need electrolytes (calcium, potassium, magnesium, sulfate, sodium, chloride). Fish have a chloride cell in their gills to pass ammonia and CO2 out and drawn in oxygen. Energy passes through cell walls using a sodium-potassium pump. Immune systems do not function as well if electrolyte levels are not maintained. For fish this is about 0.1% or 1.25 cup of evaporated sea salt for 100 gallons of water. Other salts (rock salt, mine salt, and table salt) are simple sodium and chloride. Once salt is added it does not evaporate or precipitate out. Do not add more when topping up the water level. The only time more salt needs to be added is following a partial water change. A simple titration test for salt concentration will help monitor this.
Basics of filtration. There is not a specific formula to determine filtration needs. There are three types of filtration. Mechanical filtration catches debris. If debris (floating string algae, excess food, leaves, etc.) is removed before it breaks down, water will be healthier. It does not matter what method is used if it works. If it clogs up in 24 - 48 hours, a larger filter will help reduce maintenance time. Chemical filtration removes things we cannot see. Chemical filters are often made of resin. An under utilized filter is activated carbon often used in aquariums. It removes pesticides and herbicides. Pond owners may be aware of the danger but neighbors or service companies may use these products unaware of the danger to ponds on the other side of the fence. Some cities have started pesticide-spraying programs for mosquitoes carrying West Nile virus or for gypsy moth infestations. Activated carbon will also remove oil leaking from broken pump seals. The oil is non-toxic but looks bad. Oil and floating pesticides may also be removed by Calcium Bentonite. If you notice an oil sheen on the surface of the water disperse calcium bentonite over the surface and it will capture much of it and carrry it to your filter system where it will be trapped. Zeolite removes phosphates. Biological filtration provides a home for nitrifying bacteria that remove ammonia and nitrites from water. Any material can be used that provides surface area. The more surface area, the better. Lava rock is often used but the tiny holes soon become clogged and it is impossible to clean it out well. A good rate of water flow must be maintained through the filter. There are many nitrifying bacteria products for sale but the shelf-life is short (1-2 weeks) and products must be stored at the proper temperature and pH to be effective. Plants and fish together will produce nitrifying bacteria in a pond and the roots of unpotted plants will also provide  surfaces for your nitrifying bacteria to colonize. A friend may be able to supply some from an established pond. It takes between 7 - 30 days to establish adequate bacteria in a pond. One variable is temperature. Little action occurs when the water temperature is below 50o, 70o is ideal, and some bacteria will die above 90o. A working biological system will have zero nitrite and ammonia levels. There may be ammonia spikes during spawning and if fish are overfed. Be very careful not to over feed your fish. And be sure not to overfertilize your plants. Fish will generally provide enough. Fish in a properly planted pond should not need much supplemental food

The dreaded Algae
Algae even grows in and under ice. It will continue to grow until plants begin using nutrients in the spring. Many people keep fertilizing their lilies and other flowering water plants. If you have them in pots you can see that the worst algae STARTS in the pots. Try to have as many plants as you can find that will also grow in the ice. Pennywort and watercress are two of them. Plants use nutrients and compete with algae. Plants should cover 70% of the surface during the growing season. Fish eat algae. Tadpoles eat algae. UV lights and algaecides and bentonite are also methods for control. But there is no magic bullet. It may take all of the above to get it under control but once you get established and continue the correct treatment your problems should diminish. UV is expensive and algaecide can upset the balance. But plants and calcium bentonite work with the natural pond chemistry.

If you are experiencing bad algae problems I'd suggest you go to the grocery store and buy several clumps of watercress(with the roots on) and bring it home and throw it in your pond. Tomato plants work very well too (in your filter without the pots or dirt.
And of course pennywort. If you can find it buy a large bunch and toss it in the pond. These plants suck up nutrients really fast and help starve the algae.

 

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SPECIAL AQUARIUM INFO

For small aquariums (10 to 20 gallons) just a pinch between your thumb and forefinger twice a week

100 gallons 1/2 teaspoon every two weeks

LArger aquariums calculate from table below

 

detailed spoon info

1 tablespoon per 1000 gallons twice weekly

scattering the dry powder directly on the surface of the water works best

10 tablespoons= 4.4 oz
36 tablespoons= 1 pound

100 gallon pond 1 year= 1/2 pounds
500 gallon pond 1 year = 2 pounds
1000 gallon pond 1 year= 4 pounds
2000 gallon pond 1 year= 7 pounds
3000 gallon pond 1 year=10 pounds
4000 gallon pond 1 year=13 pounds
5000 gallon pond 1 year=16 pounds
6000 gallon pond 1 year=19 pounds
9500 gallon pond 1 year=28 pounds
12000 gallon pond 1 year=36 pounds