Massive Mortality (Death) Freshwater Fish

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A quick run down of reasons why a mess of freshwater fish might be dead in the morning.

The most common cause of mass mortality in freshwater fish, like, overnight loss of most of the fish, are:

  1. Boiling by bad heaters.
  2. pH Crash is certainly the number one cause.
  3. Low oxygen levels in warm-ish water

There are very few parasites that can kill more than half of the fish overnight. So, if kills are fast and furious, (but not more than half overnight) perhaps a parasite IS in the works. Please see the SHOTGUN ideas.

Mass Mortalities Overnight (More than half the fish)

I've bought these heaters and I like them alot. I use them on my fishroom tanks stuck to the sides of the tank and readable on the side. But you can also carry one portably and use the probe to read the temp. Knowing that your tank has overheated can explain one cause of massive overnight mortalities. 
"Purchased 2 times.
You last purchased this item on January 23, 2020"

Avoiding overheating 
Use reliable heaters but back yourself up, if you set your heaters to 80 and a thermostat like the one at left, to 78, what can happen is that the heater can overheat but the thermostat will cut it off. Or, the thermostat will fail and the heater will hit the set 80. Nothing dies unless BOTH the heater and the thermostat shit the bed on the same day. 
My Amazon Account: "Purchased 5 times.
You last purchased this item on January 23, 2020"
I use these in my fishroom and at home.

This is the best heater you can buy. Titanium with the adjustable setting in the CORD so you don't have to pull it up to change it. It's 400W which means that for a smaller tank, say 20 gallons it will only be on a few minutes. And it will carry a tank up to 100 gallons. (Not against outdoor winter of course. ) I use TWO of these in my 180 gallon tank so that if one fails the other can carry. I always reduplicate things on valuable tanks and fish. 
Purchased 4 times.
You last purchased this item on January 23, 2020.
I have a total of ten of these between my home and fish room. 

Oxygen crash 
Filters stop, air pumps stop, ponds and tanks get very warm and water movement ceases. Always run two forms of water movement on two poles of power. I recall having my pump choke, and it kicked the breaker which also killed my air pump. So now I always have my airpump on one pole, and the water pump on another breaker. 
"Purchased 4 times.
You last purchased this item on October 1, 2017"
All of them are still running. 

pH Crash is the most common cause of overnight mortality of most or all the fish. Did that happen? You won't know without testing for it. Lawsuits have been won and lost over the pH. If you don't test it, then it's hard to prove WHAT, exactly, killed all the fish. 

So skip the pH Crash. Just buffer the pH not high, not low but RIGHT ON neutral. It's easy. At left, 2.2 pounds of Neutral Regulator. In a large fish tank that will last a long time. But they have BUCKETS of the stuff, too.