Why Spring Kills More Pond Fish Than Any Other Season

We hear about more fish deaths in spring than at any other time of year. And almost every time, the cause is the same: the pond keeper did exactly what instinct told them to do, and it triggered exactly the conditions that kill fish.

This isn't bad luck. It's biology. Once you understand what's actually happening inside your pond during the first warm weeks of the year, the reasons are completely logical — and completely preventable.

What Is New Pond Syndrome?

New pond syndrome is the name given to the water quality crash that happens when a pond is first set up. The fish go in before the biological filter has had time to establish itself, ammonia and nitrite build up to toxic levels, and fish suffer or die.

The reason it's relevant in spring is this: an established pond in early spring can behave almost identically to a brand new one.

Over winter, your biological filter has been running at a fraction of its normal capacity. The nitrifying bacteria and archaea that convert ammonia into nitrite and then into the relatively harmless nitrate are temperature-sensitive organisms. Below roughly 10°C, their activity drops sharply. By midwinter, the system is barely ticking over — but that's fine, because the fish are barely producing waste.

The problem comes in spring, when water temperatures start to climb.

The Filter Lag

Here's the critical mismatch: your fish wake up before your filter does.

As water temperature rises, the fish's metabolism accelerates first. They become more active, produce more waste, and — if you're feeding them — start generating significant ammonia load. The biological filter, meanwhile, takes time to rebuild its bacterial populations and ramp back up to full nitrification capacity.

The result is an ammonia and nitrite spike in an established, seemingly healthy pond. Exactly like a new pond. If you're not testing, you won't know it's happening until you start losing fish.

Why Ammonia Is More Dangerous in Spring

This is the part most pond keepers don't know, and it matters a lot.

Ammonia exists in two forms in pond water: ionised ammonium (NH₄⁺), which fish can tolerate in relatively small concentrations, and un-ionised ammonia (NH₃), which is directly toxic to fish at very low levels. The ratio between these two forms is controlled by pH and temperature.

The higher the pH, the more of your total ammonia reading is in the dangerous un-ionised form. And in spring, pH rises — often dramatically — every afternoon.

Most people test their pond water in the morning when pH has dropped overnight — and completely miss the dangerous afternoon spike.

The reason for the afternoon spike is photosynthesis. As algae and pond plants photosynthesize during the day, they consume dissolved CO₂ from the water. CO₂ acts as an acid in solution, so removing it causes pH to rise — sometimes from a safe 7.5 in the morning to over 9 by mid-afternoon. At pH 9, a level of total ammonia that read as harmless on your morning test can be genuinely dangerous.

The foundational work on this relationship was published by Emerson et al. in 1975 and remains the basis for modern ammonia toxicity calculations. The interaction of pH and temperature with ammonia speciation is well established — and it means that spring pH dynamics significantly amplify the risk of even moderate ammonia levels.

Green Water: What It Actually Means

Green water is caused by a bloom of free-floating single-celled algae (phytoplankton). It's extremely common in early spring, when nutrients are high from winter accumulation, water is warming, and daylight is increasing — ideal conditions for rapid algal growth.

Many pond keepers treat green water as a cosmetic problem and reach for a UV steriliser. UV will clarify the water by killing the algae cells, and it does work — but it doesn't address the underlying cause: elevated nutrients. Turn the UV off, and the bloom returns.

Green water in spring is also an indicator that your biological filtration hasn't yet caught up with the nutrient load in the pond. The algae are exploiting exactly the same ammonia and phosphate that your filter bacteria should be processing. It's a symptom, not the problem itself.

The Single Most Damaging Spring Mistake

The logic seems sound: the filter has been sitting there all winter, it must be clogged up, let's give it a good clean before the season starts. But tap water contains chlorine and chloramine — added specifically because they kill microorganisms. Running your filter media under the tap doesn't clean it, it sterilises it. You are removing the biological filtration you spent months building.

If you need to clean filter media, rinse it gently in a bucket of pond water removed during a partial water change. That's it. The filter should smell earthy or slightly musty, not sterile. Leave the biology alone.

How to Bring Your Pond Out of Winter Safely

• 1
  Test before you do anything else

  Before you feed, before you clean, before you adjust anything — test ammonia, nitrite, nitrate, and pH. You need to know what you're working with. If ammonia or nitrite is showing, treat cautiously and hold off on feeding.

• 2
  Don't start feeding until the water is consistently above 10°C

  Not one warm afternoon — consistently, including the morning reading when temperature is at its lowest. In deeper ponds, temperature stratification means the bottom warms last; that's where your fish have been sitting all winter. Start with small amounts of wheatgerm once a day, watch what happens, and increase gradually only if food is disappearing cleanly.

• 3
  Leave the filter alone

  Do not clean your filter with tap water under any circumstances. If the media needs a rinse, use a bucket of pond water. The biological colony in your filter is your most valuable piece of pond equipment — protect it.

• 4
  Do a modest partial water change early in the season

  A 10–20% water change in early spring helps dilute accumulated nutrients — nitrate, phosphate, organic load — before the algae and bacteria get to work on them. This reduces the bloom intensity and gives the filter a more manageable starting point.

• 5
  Read your fish correctly

  Fish actively swimming at all depths and showing coordinated foraging behaviour is different from fish drifting to the surface on a warm afternoon. The former suggests the metabolism is genuinely coming back online. The latter is just a temperature response. Watch for clamped fins, lesions, loss of balance, or surface gasping — these are your early warning signs that water quality has deteriorated.

A Note on Spring Disease

Spring is also the high-risk season for disease outbreaks in pond fish. Cold temperatures suppress the immune response over winter, and as fish warm up and become more active, any parasites that have been quietly present suddenly encounter a host whose immune defences are only just coming back online.

Keeping feeding conservative in early spring reduces the metabolic load on fish that are already immunologically vulnerable. We'll be covering spring disease in detail in a separate video.

Feeding Temperature Guide

Water Temperature	Feeding Recommendation
Below 10°C	Do not feed. The digestive system cannot process food safely at this temperature.
10–14°C	Wheatgerm only, small amounts once daily. Watch carefully and remove any uneaten food.
14–18°C	Begin transitioning back to a standard diet. Mix wheatgerm with a quality pellet and increase the ratio gradually.
Above 18°C	Feed normally. Higher-protein food is appropriate again as the metabolism is fully operational.

Use a thermometer, not the fish's behaviour, as your primary guide. Fish will beg even when they shouldn't eat. Check temperature in the morning, before solar warming has raised the surface reading, and measure consistently at the same depth and location each time.