Your air pump
isn't pumping oxygen
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The full narration of the video.
That stream of bubbles in your fish tank puts almost no oxygen into the water. The air pump pushes air down a thin tube and out of the air stone on the bottom. The bubbles float up, reach the surface in a second or two, and pop. That trip is far too short. Of every hundred bits of oxygen in a bubble, two or three dissolve at most; the rest goes straight back into the air.
Yet in a crowded tank, switch the pump off for long and the fish really do come up gasping. So where does the oxygen actually get in? Air is full of oxygen: twenty-one of every hundred molecules. In water, it's only about ten in every million. Oxygen drifts from where there's more to where there's less, but water and air meet in only one place: the surface. Most of a tank's oxygen comes in right there.
The catch: that thin top layer fills up fast, and once it's full it stops taking any more. If the water sits still, the top is rich in oxygen while the bottom goes short. It's like a pot of soup fresh off the stove. Soup only loses heat from its surface, so left alone the top cools while the bottom still scalds; give it a stir, the hot soup rolls up, and the whole pot cools much faster. That stream of bubbles is the tank's spoon. Rising bubbles drag the water around them upward; at the top it spreads out, soaks up oxygen, then sinks down along the glass. Oxygen-poor water from the bottom gets pulled up next.
Round and round, the whole tank takes turns at the surface. Bursting bubbles keep the surface shaking, so the full layer keeps getting swept aside, and the swap goes faster. This is called gas exchange. The carbon dioxide your fish breathe out leaves through the same surface. Look right above the air stone: that patch of water keeps bulging and churning. That's the spoon at work. So if your filter outlet keeps the surface moving, the fish will usually be fine even without the pump. But if floating plants cover the whole surface, the pump can roar all it likes and do far less.
Next time you feed them, try this: drop a pinch of food beside the air stone and watch it ride up, drift to the glass, then slowly sink. That's the loop of water at work. Do you switch your air pump off at night?
A bubble spends a second or two in the water
Of a hundred bits of oxygen,
two or three stay at most.
The air pump pushes air down a thin tube and out of the air stone on the bottom. The bubbles float up, reach the surface in a second or two, and pop. That trip is far too short, so most of the oxygen goes straight back into the air.
A home tank is 30–40 cm deep. Even fine-bubble diffusers covering a treatment-plant floor dissolve only about 6–7% of their oxygen per metre of clean water; in a tank this shallow, that works out to 2–3% at most.
Oxygen comes in at the surface
Twenty-one of every hundred molecules in air are oxygen. In water it's only about ten in every million. Oxygen drifts from where there's more to where there's less, but water and air meet in only one place: the surface.
The catch: that thin top layer fills up fast. If the water sits still, the top is rich in oxygen while the bottom goes short.
Like stirring hot soup
Soup only loses heat from its surface. Left alone, the top cools while the bottom still scalds; give it a stir, the hot soup rolls up, and the whole pot cools much faster.
That stream of bubbles
is the tank's spoon.
How the bubbles stir
Round and round, the whole tank takes turns at the surface. Bursting bubbles keep the surface shaking, so the full layer keeps getting swept aside and the swap goes faster. This is called gas exchange.
So watch whether the surface moves
In a crowded tank or on hot days the water holds little oxygen to begin with, so keeping the pump on is the safe call.
Try this at feeding time
Drop a pinch of food beside the air stone and watch it ride up to the surface, drift to the glass, then slowly sink. That's the loop of water at work.
air pump off at night?
Quick check
Where does most of the oxygen in a fish tank get in?
Bubbles reach the top in a second or two and keep only 2–3% of their oxygen. Their real job is stirring water up to the surface.
