The Effort to Breathe When our rib case or stomach muscles expand our lungs, air immediately flows into our body. We breathe 'en-passent' while a fish has to breathe the much more viscous water as it they were terminally ill asthma patients.
They use about 50% of their energy to pump water through their gills.
Energy consumption requires oxygen and so a fish uses up quite a lot of it to obtain relatively little more. This is a constant battle.
In free range ponds this is - apart from in winter - normally not a problem. Things become critical when the fish are forced into activity under adverse circumstances.
Avoid Winter Stress
A lack of oxygen can occur when ice skaters are on the ice. In a thusly irritated carp oxygen consumption can skyrocket to 300 times its normal rate due to stress and hectic. In this one hour it can use up the oxygen that would've otherwise lasted for two weeks. We had a number of losses because of excavators working in 50m distance.The oxygen supply can easily be ensured through pumps, filtration technology and air stones. With very dense fish population we always recommend an additional warning or emergency system. Customers have reported about tons of lost fish that very often were due to small operating faults.
Combined Problems Less known is the fact that the oxygen level alone says little about the way it can be used and what effects it has in the body:
Low levels of oxygen are especially dangerous in combination with high pH and low CO₂ concentration. This leads to alkalosis. A lot of oxygen and CO₂ in conjunction with low pH levels leads to acidosis.
Extreme oxygen over-saturation of 300% leads to initial symptoms of illness, from 600-700% over-saturation oxygen is nearly always fatal - often in less than an hour!
Other oxygen problems appear indirectly: A lot of oxygen makes the initial colonisation of biological filters impossible. Note that these systems can only start up quickly when the water is under-saturated with oxygen!
