Why Won't My Fish Eat?

Why a Fish Might Not Be Eating

It can be concerning when a fish refuses to eat, especially if it is new to your aquarium or has previously been eating well. Appetite loss in fish can happen for many reasons, and identifying the cause is the first step toward resolving the problem.

Stress and Appetite Loss

The most common question when a fish isn't eating is, "What foods has this fish been feeding on?" Unfortunately, when a fish is experiencing appetite loss, it's not because of the food types offered. The most common reasons fish stop eating is stress. Stress can come from shipping, environment, sudden changes in water parameters, aggressive tankmates, poor water quality, or even frequent disturbances around the tank. When fish are stressed, their bodies focus on survival rather than digestion. This physiological shift often results in appetite loss until they feel secure again. Giving new arrivals time to adjust, keeping water parameters stable, and minimizing unnecessary stressors can help encourage feeding.

Environment & Hiding Places

If a fish is in a too-small tank or container, or if they don't have enough hiding places, this will affect their appetite. If you're keeping a fish in an acclimation box, ensure it's large enough for the fish you're keeping. Also consider that some fish species are more sedentary, while others are agile swimmers and may need more room. It's imperative to give your fish a small hide like a PVC pipe long enough and wide enough to conceal them, or a plastic plant decoration they can hide in. 

floating plant for hiding from Chewy.com

Water Quality and Environmental Issues

Poor water conditions or a fast change in water parameters (even when they're still "in range") can quickly cause a fish to stop eating. Ammonia, nitrite, and high nitrate levels, as well as incorrect/swinging salinity, pH, or temperature, can all have a major impact on appetite. Even if the fish is disease-free, being in suboptimal conditions can cause lethargy, rapid breathing, and food refusal. Testing your water frequently and keeping up with regular water changes is important. It's also important to re-calibrate your water quality measuring tools and check the expiration of your test kits before testing your water.



It's very common for fish in quarantine or uncycled aquariums to experience appetite loss for these reasons. Even if the tank has been set up for a while, a quarantine tank is different from a mature display tank with a lot of biodiversity. Even if the basic water parameters like ammonia are "good," the tank can suffer from swinging water parameter values like pH and ORP, which can be stressful especially for sensitive species. We have an article here about keeping a quarantine tank safe for new fish: https://shop.thebiotagroup.com/blogs/news/the-secret-to-a-healthy-quarantine-aquarium

Ammonia Burn

Ammonia is very toxic to fish and can burn their gills, eyes, skin, and in extreme cases, causes adipose tissue loss and alarming, rapid weight loss. Ammonia causes appetite loss for a number of reasons, mostly because of the stress on the body and organs, and because the irritation can make eating painful. Though hardy species may briefly tolerate trace amounts of ammonia in an under-cycled or quarantine tank, more sensitive species like Regal Angelfish can not tolerate even trace amounts of ammonia that may not even be high enough to show up on your hobby ammonia test result. That's why proper quarantine and making sure the tank is mature and stable before introducing fish are so important. 

Food Preferences and Palatability

Some fish are especially sensitive to changes or issues with water quality and may experience appetite loss during transitions: fish like seahorses, mandarins, and sensitive angelfish. Even if you offer the food types they've been raised on and are most familiar with, they likely won't eat until the cause of their stress is addressed and resolved. In these cases, sometimes offering natural foods, like in the case of angelfish, rockwork with a lot of algae for grazing, or live shrimp for seahorses, can help the fish keep weight on until the stress resolves and they resume eating the prepared foods they were raised on again. 

Fish have a great sense of taste and smell, so they may reject foods that are not fresh. Foods with a high fat content like Mysis that have Highly Unsaturated Fatty Acids (HUFA), frozen foods, and foods with a high water content are particularly prone to spoilage. Fish can tell when a food has gone rancid, and especially sensitive species will avoid it.

If you're treating your aquarium water with medications or mixing medications or additives into your fish food, this can also cause fish to reject food because of the taste. If the medication is necessary, continue to introduce the fish to it, and take short breaks to make sure they're getting enough unmedicated food to maintain body condition if needed.

Illness, Injury, or Parasites

Parasites, bacterial infections, and other health issues can also cause a fish to stop eating. Gill infections, like Cryptocaryon (ich) or bacterial infections can make it uncomfortable for a fish to eat, sort of like when humans have a sore throat. 

A fish with a sunken belly, weight loss despite eating, or abnormal feces may be dealing with an internal parasite. This is most common in wild-caught fish or captive-bred fish housed with wild-caught fish. Quarantine and treatment may be necessary in these cases.

Fish with mouth or gill injuries may also lose interest in food. Keep a close eye on the injury for signs of infection like redness or swelling. Offer smaller, softer food items while the fish is healing. 

Acclimation Period for New Fish

Newly introduced fish often go through an acclimation period where they may not eat for several days. The stress of transport, new surroundings, and different tankmates can suppress appetite temporarily. This is normal as long as the fish does not show signs of illness, injury, or significant weight loss. During this time, keeping lighting gentle, limiting handling, and offering a variety of familiar foods can help them settle in faster.

We recommend allowing a new fish to settle in with the lights on the day of delivery. Begin feeding the new fish in the morning. New fish or young fish should be fed small meals at least 3 - 5 times a day. Feeding frequently is essential for their fast metabolism and recovery from shipping, and it gives the fish more opportunities to eat if they are experiencing mild appetite loss. 

Water Flow & Pavlov's Response

Newly introduced fish, young fish, or species that are naturally poor swimmers like filefish may have a difficult time seeing or chasing food if all the wavemakers and pumps are on during feeding time. We recommend turning all your pumps to "feed mode" before feeding. We do this in our facilities, and we notice that it trains the fish to anticipate that when the pumps turn off, it's time to feed!

Why Fish Sometimes Spit Out Food

Many aquarists are concerned when they see a fish take food in its mouth and then spit it out. While this can be a sign of a problem, it is not always cause for alarm. Sometimes fish do this simply to break food into smaller pieces before swallowing. In other cases, it can be a sign of a problem.

1. Food Quality
If the texture or flavor of the food is not appealing, the fish rejects it. This can happen if the food is not fresh or is an unfamiliar type.

2. Reduced sensory response
Stress hormones can dull their sense of taste and smell, so food that normally seems appealing might suddenly taste "off" to them.

3. Disrupted digestion
Stress triggers a fight-or-flight response. Blood flow is diverted away from the gut toward muscles and gills, which slows digestion. The fish may instinctively reject food it doesn’t feel ready to process.

4. Caution behavior
Many fish will pick food up to inspect it, then spit it out if something feels unusual, this could be due to different lighting, water chemistry changes, or even the smell of medications in the water.

5. Gilling due to poor oxygen exchange
If stress is paired with low oxygen or gill irritation, chewing and swallowing can feel uncomfortable. They’ll grab food, then quickly spit it so they can resume rapid breathing.

6. Gut slowdown after fasting or shipping
If they’ve gone a while without eating (such as during transport), their digestive enzymes may not be ramped back up yet, so the body rejects the food until it’s ready. We recommend waiting until the morning after arrival to resume feeding again.

Conclusion

A fish refusing food can be due to environmental stress, water quality problems, illness, or simply needing time to adjust. Observing behavior, testing water parameters, and offering a variety of food options can often resolve the issue. If appetite loss persists or the fish shows other symptoms, further investigation and possible treatment may be required.