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:
- Boiling by bad heaters.
- pH Crash is certainly the number one cause.
- 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)
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.
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.