Sorry, I am a rambler, I just ramble on.
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If ya really want to do a water change, then a safer way is a "trickle water change". Google it. It accomplishes the water change in an extremely slow pace by connecting your water outlet to some drip emitters (stuff that is used for garden irrigation). The old water exits through an outflow. I am told it is beneficial since the exchange happens so slow that only an extremely small percent of your pond water reacts to it so you, fish, and plants will not notice side effects.
The drip emitter output volume determines the water change. Example: There are 168 hours in a week. A Netafim .26 gallons per hour emitter over a week puts out (.26 x 168 ) 43.68 gallons of water. If your pond is very small like my water feature, 435 gallons, then a .26gph emitter gives a 10% weekly water change. Evaporation not taken into consideration. If ya want to take into account of evaporation, then step up to using a .5 gph or two of the .26gph emitters.
Netafim is a specialized drip irrigation material used for underground irrigation for lawns or above ground irrigation for gardens. You will likely have to go to a specialized landscaper business to buy the stuff. There might be other drip irrigation manufacturers that goes down to .26gph, but ya can always find the more common .5 or .9 or 1 or 2 gph emitters. Netafim is bought in rolls so ya can use the remainder on your trees or flower garden. Netafim is expensive. I use Netafim everywhere around my house so I have a few rolls of it. Netafim is different since the emitters are actually
inside the tubing and the emiters are built to prevent clogging by a special air cavity and cage inside the emiter in the tubing. Ya can use other drip irrigation material.
I like the Netafim emiters since it allows me to put the drippers inside a pvc pipe, using a selection of pipe reducers, then this pvc pipe goes into the bottom of my water feature. I will do a writeup how I did mine once it is complete. I still have some more work to do on it. I am also going to insulate this and put a heat trace wire so I can run it during the winter.
Your humidity will be a big determination of your water evaporation. Here where I live in Texas, our humidity is anywhere from 0% or up to 90% if it is going to rain. In the 100 degree days with 3% humidity and high winds, with my waterfall, stream, and fountains, I have seen my water feature lose up to 60 gallons in one day.
So, ya need to build the trickle system so ya can easily change the emitters to adjust to the weather for evaporation.
I can't rely on rain. Heh, year of 2011, my house only got 3 inches of rain the entire year. Normally, we get 8~11 inches annually but don't tell the farm land real estate guys; these guys still tell buyers we get up to 12~15 inches of rain, lol. To dryland farm, ya need at least 15 inches of rain; dryland farms exist with 10 inches of rains only if they are lucky enough to be in the exact right spot receiving the rain and if it rains when the crops need it. If anyone is curious how we survive farming, it is due to the Ogallala aquifer and us pumping anywhere from 400~600 gph into our sprinklers for growing season.
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From my first few weeks of using water from my well, I noticed that my hard water keeps my KH quite high. KH is the energy source for the nitrification process (google nitrification process); as ammonia is produced by fish and decay, KH is consumed for the nitrification process. If your rain is soft (has a low pH rating), then it can eat up your KH. Eventually, you will have to add baking soda or crushed oyster shells or Mother Nature will have to add something to your pond to raise your KH. KH also buffers your pH so your water does not become too acidic. My water feature is so small and under some cedar elms so I bet Mother Nature does not add much other than dirt and bird poo and very little rain. My well water replenishes my KH. So, I am sure I will never have to add any of the oyster shells or other stuffs. If your pH is really low, then likely your KH will be very low as well (as far as I am aware).
GH is one of the main indicators for how much minerals ya have in the water, and GH is also a pH buffer so that the water does not become too alkaline.
API is a brand name of a good liquid test kit that provies a GH / KH test kit.
Many folk, more than others less fortunate, have many thanks to give to Mother Nature for what she provides so you maybe one of the many that will never have any of these problems.
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I know minerals are good for Koi. I have read how Koi actually eat the clay in mud ponds and this is how they get their minerals since the water in these mud ponds are in the 6.5~7 pH range.
Some plants don't like minerals so much. More hardy legumes and other plants can tolerate the minerals. Hydroponic gardeners often use Reverse Osmis water due to the lower GH. Minerals bind to nitrogen, potassium, phosphorus, and other nutrients slowing down the acid breakdown process of raw to soluble nutrients allowing the plant to uptake the nutrients. The soil in my area is extremely alkaline, pH 8~9. Most of our trees suffer from severe iron deficiencies due to the alkaline soil binding the raw nutrients. I farm for a living and we often have to add extra volumes of Sulfur to our fertilizers to reduce the alkalinity so that it is easier for all of our crops to uptake the fertilizer and other nutrients in the ground.
Don't get me wrong. Alkaline soil and alkaline water
will not stop the production of vegetable and other type of fruits. It just slows down the plant and your yield will suffer. Blueberries will provide average foliage but will rarely produce fruit in alkaline soil/water, but there are alkaline loving berries such as a variety of cherries.
Now, when it comes down to pond plants, foo foo pretty perenials or annuals. I know nothing, heh.
