I am likely missing some references. Grr, I got 50 or more bookmarks. I am gonna start organizing them. Sucks having to search for everything everytime I am wanting to share the info.
So, this is what I know at this moment ... always learning from corrections and more reading
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Weather here has stayed a few hours in the 102~105* F for 5 days and our lo-temp never dropped below 68~70*F. My water temp has been 78~80*F. I have just a couple air diffusors. Also, most of my water is pulled from the bottom, where it is cooler, to the top; this probably keeps the temperature up. Also, the water is in partial shade due to big cedar elm trees preventing the water from heating up much higher.
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In northern Iowa we have some of the best trout streams in the area. The DNR doesn't put bubblers in the streams, there are waterfalls and fast moving water at least every 1/4 mile of stream. Trout must have cool water with high O2 levels.
I bet the stream water is very cold. Trout are cold blooded and species of Trout dictate DO demand. Google "trout dissolved oxygen". I bet those streams are not closed systems. I bet open system streams constantly have the water refreshed with the highest possible O2 saturation at all times unless there is a dead zone in the stream.
Water temperature is not the same at all depths. Water temperature dictates maximum O2 saturation. Cold water dictates very high O2 saturations. Warm water dictates very low high O2 saturations.
Koi, comets, and many other hardy fish are cold blooded fish. Cold blooded fish's DO demand is dependent temperature since their body metabolism is mutually exclusive with water temperature. Feeder goldfish (also called comets) don't need much O2. You can often find comets in algae covered sections of lakes. Comet, feeder goldfish often found to live in dissolved oxygen (DO) levels of 5mg/L where as more demanding fish such as an adult Koi need 8mg/L or higher.
Koi grow very fast. So, their DO demand is obviously higher when the water temperature increases their metabolism. Pretty simple. Comets are much more hardy since they grow extremely slow.
Dissolved Oyxgen - Koi Club of San Diego
maximum O2 staturations for freshwater
90* F water = 7mg/L DO
70* F water = 9mg/L DO
45* F water = 12mg/L DO
35* F water = 15mg/L DO
Salinity further decreases possible saturation levels.
Floating plants ... Think about this and how atmospheric pressure adds O2 to water. Rivers become severely oxygen depleted due to floating plants. Floating plants may help cool the water, but they also help deplete O2 levels. Do a google search about when floating plants become dangerous, killing aquatic life.
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I know that you "think" that a bubbler is the best way to get O2 into water, but from trial and error, breaking the surface of the water with a waterfall or something of that sort works.
Waterfall pushes the top O2 saturated water to the depths of the water where water is less oxygenated. Sheet waterfalls pushes oxygenate water down and creates small bubbles that help stir the water. Cascading waterfalls only oxygenates water at a very shallow depth.
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Ammonia Toxicity increases with water Temperature.
