Fish that eat but don't grow: feed, oxygen, and FCR
When oxygen is marginal, animals eat but do not convert. Below half-saturation, appetite and digestion fall, and the stock burns energy coping with stress instead of turning feed into weight. Feed conversion ratio (FCR, the feed it takes to add a kilogram of animal) climbs, and feed is the biggest cost you carry. Steadier oxygen returns that energy to growth.
What’s actually happening in your water
Fish and shrimp are cold-blooded, so the water sets the pace of their metabolism, and oxygen fuels it. Above a comfortable level of dissolved oxygen (the oxygen dissolved in the water), the animal eats well, digests efficiently, and puts most of that energy into growth. Let the oxygen drift into the marginal range and each of those steps loses ground.
Appetite falls first. Below roughly half of saturation, the point where the water holds half the oxygen it could, the stock feeds reluctantly. What it does eat, it digests less efficiently, and a larger share of the energy goes to simply coping with the stress rather than adding weight. The result is an animal that keeps eating on the feed schedule while converting less of it, so the feed conversion ratio (FCR, the kilograms of feed it takes to add a kilogram of animal) climbs.
There is a second turn of the screw. Feed that is offered but not eaten sinks and decomposes, and that decomposition consumes oxygen of its own. So marginal oxygen both raises the feed you spend and adds to the demand that lowered the oxygen in the first place.
Why the usual fixes don’t hold
The instinct is to feed harder to chase the growth. That usually backfires: more feed the stressed stock will not eat means more waste on the bottom, more decomposition, and more oxygen demand, which pushes conversion the wrong way.
Adding daytime aeration misses the hours that matter. Conversion suffers most through the night and the pre-dawn low, when a surface aerator running in the afternoon has already done its work. The stock needs oxygen held through the marginal hours, not a burst at the wrong time of day.
How restoration works here
Nanobubbles stay suspended and carry oxygen through the whole water column, continuously, including the overnight hours and the bottom water. Held above the stressful range around the clock, the stock feeds normally and digests well, so more of every meal becomes weight instead of being spent on stress.
The proof is on records you already keep. We install the system, and Stewardship logs dissolved oxygen continuously while you log feed and growth, so feed conversion before and after can be read against one baseline. What we measure and how is published, because a claim about your feed bill should be a number you can check.
What to expect, and when
Weeks 2-4
Dissolved oxygen holds higher through the day and the night, and the feeding response steadies. Growth is slow to read this early, so we track the oxygen the growth depends on.
Weeks 6-12
With oxygen no longer dropping into the stressful range each night, more of the feed goes to weight. Feed conversion begins to reflect it on the feed records you already keep.
Season 1
Over a full grow-out, feed conversion and growth rate against the baseline are the record. Feed is the largest cost, so the number that matters is kilograms of feed per kilogram of gain.
The record
We don't have a published case file for this problem yet. Every Alchemal installation is instrumented from day one, so the first case files are being measured now, and until one is ready, our methodology shows exactly what we record and how we report it.
When this isn't the right fix
- Oxygen is one driver of feed conversion, not the only one. Feed quality, genetics, water temperature, and disease all move it. If oxygen readings are already strong, the assessment looks elsewhere rather than selling you oxygen you do not need.
- Overfeeding reads as poor conversion too, and no amount of oxygen fixes feed that lands on the bottom uneaten. Feeding practice is part of the review, not an afterthought.
- If growth stalled suddenly rather than drifting down, suspect a health or water-quality event rather than chronic low oxygen. We help you tell them apart before proposing anything.
Questions people ask
Why are my fish eating but not growing?
Eating and growing are two different steps, and low oxygen breaks the second one. When dissolved oxygen sits in the marginal range, the stock digests less of what it eats and spends more energy just coping, so less of the feed becomes weight. The feed goes in, the growth does not come out, and the feed bill keeps running.
How does low oxygen raise the feed conversion ratio?
Feed conversion ratio is kilograms of feed divided by kilograms of gain, so anything that reduces gain for the same feed pushes it up. Marginal oxygen suppresses appetite, slows digestion, and diverts energy to stress. The animal converts a smaller share of each meal, and the uneaten feed decomposes and consumes more oxygen, which deepens the problem.
What is a good FCR, and can oxygen improve it?
A good ratio depends on the species and the system, so the useful comparison is your own pond against its baseline, not a headline number. Holding oxygen out of the stressful range removes one real drag on conversion. It is not the only lever, which is why we measure the change rather than promise a figure.
Do the animals eat less when oxygen is low?
Yes. Appetite falls as dissolved oxygen drops, and once the water spends hours below roughly half of saturation the stock feeds reluctantly and digests poorly. Because the daily low lands before dawn, the hours around it are when feeding and conversion suffer most, even if the afternoon reading looks fine.
Will better oxygen alone lower my feed bill?
It removes one of the largest drags on conversion, which tends to move the feed bill in the right direction, but feeding practice and feed quality sit alongside it. The way to know is to measure feed conversion before and after against the same baseline, which is what Stewardship records.
Tell us what your water is doing.
A specialist reads your description and replies with a plain answer: what it usually means and what we would measure first.