Excess pearling after water changes - another idea


Guru Class Expert
Jan 12, 2007
Dorset, UK
Hey all,

While staring at my tank last night, having previously just gassed my fish and had to carry out an emergency water change, I had this idea of why all our plants pearl much more in the 12 hours after water changing.. see what you think, its just a theory, and might be full of holes.

Pearling happens when the plant is producing more O2 than can be effectively dissolved into the water column.

When we add fresh water, the water we put in is pretty super-saturated with O2.. so could it be, that under the minimal pressure of a fish tank, the saturation levels are just too high in the water to accept any more O2 from the plants?

Using my scuba knowledge about partial pressure and gas theory, to super-saturate a liquid with a gas, a higher partial pressure can be achieved by putting the liquid under pressure, much like a soda bottle.. when you release the pressure on the soda bottle, the gas (in this case co2) come out of solution and turns back into the gas phase. This is what happens when a diver comes up to fast, and nitrogen comes out of solution inside his/her body, causing micro bubbles in the nervous system.

Could this be whats happening when weve added fresh o2 laden water? The partial pressure of the O2 is purely to high to accept new gas from the plants.. so it remains out of solution as bubbles...

so.. if this is correct.. the excess pearling we see when weve carried out a water change, isnt actually a growth spurt, its purely o2 with nowhere else to go.

With no o2 measuring equipment i cant test this theory.. although it stands up on paper.

This would also explain why it takes a short while for the plants to start pearling when lights go on. The O2 levels in the water would have stabalised over night with no fresh addition of O2 from the plants (surface movement cant super-saturate).. after a few hours, the O2 levels rise to capacity, and the O2 from the plants no longer has anywhere else to go, so it bubbles off.

Tom Barr

Staff member
Jan 23, 2005
The water is not saturated with CO2 or O2 for that matter.

Whether it's with plants, new tap etc.

I think what causes confsion is that folks think that ambiment air pressure levels, that is 100% of that, but the water can hold a lot more gas than that.

When super saturation is talked about, it is only relative to the partial pressure in air, it's not that it cannot hold more than 100% and suddenly forms bubbles and degases.

If you think about it, you would see the CO2 gas doing the same thing even though it's 10X that of air level saturation, while O2 is at best 1.5X more.

Either way, you can let the tap water sit for a bit, then add it to the tank in the morning. Then wait till the end of the night before the lights go off. By then, the gas is equilibrated.

If you compare the pearling then vs the day before, you'll see a large difference.

Tom Barr