Keeping Your Freshwater Fish Healthy

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An inexpensive isolation tank comes in handy for all aquarists. Josh Highter

Preventing & Stopping Common Fish Diseases

By Mary E. Sweeney

Much of the time disease in the aquarium is, not to put too fine a point on it, a disaster. The trouble starts with a new fish or fishes being introduced to the established aquarium. The next thing, there’s a spritzing of spots or a tilting fish, and before you know it, the population starts to decline. Without appropriate intervention, the more delicate fishes are all dead, and before long, the hardier fishes are gone as well. What’s to be done?

First and foremost…

This cannot be emphasized strongly enough nor often enough: quarantine all new fishes before introducing them into your healthy aquarium. How many piscine souls could have been saved throughout fishkeeping history if only we could drill this into our heads! (I could not say this so emphatically had I not had so much personal experience over the years…) By keeping new fishes in a separate tank for even as little as a week—although 30 days is highly recommended—you can avoid spreading virtually all of the contagious diseases to the fishes you already have. Also, it’s easier to treat diseases, whether they occur in your home aquarium or in a new acquisition, in the unadorned quarantine tank. Think about it the next time you’re planning a fish purchase.

Healthy guppy pair in a peaceful environment.

When treating your sick fish, the quarantine tank becomes the hospital tank. It is difficult and often impossible to successfully treat fishes in a show tank. Concerns over filter bacteria viability, plants and medication use, disease vectors in the aquarium, gallonage, and many other variables, even to the amount of mulm in the substrate, make it risky to use medications in the community aquarium. It is far more effective, when treating fishes, to use a simple temporary setup. You can use a small aquarium (whether a clean, fish-only bucket, plastic storage container) of perhaps 10 gallons—depending on the size and number of fishes to be treated—a heater, thermometer, and a sponge filter with perhaps an extra airstone. Nothing else is really required except some cover for the fish to hide in, under, or behind, as is their nature. Different strokes for different species. The vertical plastic plants that make an angelfish feel invisible would translate into the underside of a log for a pleco or a rocky cave for an mbuna. If you must treat the fishes in the show tank, start with a 50% water change, clean the gravel very well, and increase aeration before proceeding with carefully selected medications.

Be willing to accept success at any point in the process. If, after performing the water changes and adding salt, the fish are not as skittish or their color has improved, consider that this has altered the course of the disease process and be willing to abandon the use of antibiotics, dyes, or other chemicals.

Salt Cures

Salt is an extremely effective curative in the freshwater aquarium. I don’t believe that freshwater fishes should be kept in salted water, unless they are truly brackish-water fishes, and certainly there are some fishes, generally scaleless, that don’t respond well to salt, medications, or dyes, but these exceptions aside, common salt used as a dip or a bath is often enough to eliminate the need for further medication. It is known to destroy many kinds of bacteria, to increase slime-coat production (a positive thing), and even to cause some parasites to drop off. It can also serve as an all-round tonic, but do not mistake that for a reason to keep freshwater fishes in a constantly salt-enhanced aquarium. This brings us to another point on the health-disease continuum in the aquarium. Fish often react to incorrect temperatures, out-of-whack pH or alkalinity, poor lighting, wrong or insufficient foods, hostile tankmates, bad water chemistry, and certain other unnatural conditions in one simple way: they get sick.

The Stress Factor

The most frequent cause of fish loss is not disease as such, but stress. While we may not have much sympathy for the overachiever who complains of their own self-induced stress, the fishes we adopt into our homes are at our tender mercies. We must be alert to stressors in their environment if we are to keep them healthy. The aquarium should be placed in a quiet area so the fishes are not constantly bombarded by human activities. All of the fishes must be compatible. If a fish is constantly being chased, it will surely die. It may not die from wounds inflicted by the other fish, but the stress of being pursued will diminish not only its standard of living but the duration as well.

Water chemistry, quality, and temperature must also all be within normal limits or the fish will be stressed and stress will eventually kill them outright or set them up for the various disease processes that will eventually do them in. There are many sources of stress in the life of a fish, from being chased by a net to people tapping on the glass of the aquarium. If the fish hides or flees constantly, we can be fairly certain it is highly stressed. If the fish are trying to escape from the water onto dry land, you can form your own conclusions.

Water Quality

Polluted aquarium water is a certain cause of disease. Toxins in the water kill fish. Rotting plants, excess food, and fish waste all pollute the water. Dead fish definitely pollute the water. Polluted water is full of bacteria and fungus that will be happy to move uptown from their homes on fish waste to reside on the fish itself. Such is the cycle of life and death in the aquarium. When the aquarist becomes complacent and the partial water changes become less frequent, the stage is set for various diseases to show up in the tank. The simple act of adding another fish—even a healthy fish—can alter the delicate balances of the aquarium, and there’s your next disease outbreak. Regular water testing is essential to maintaining high water quality.

When ammonia, nitrite, nitrate, or phosphate levels begin to rise, there are several remedies. More frequent and larger changes with pure aged water should be performed. Larger than normal water changes in the 25-50 percent range are necessary as otherwise the dilution and removal of these nutrients is negligible and the problem lingers or becomes worse. If you are feeding more than your fishes are eating, it may also be time to use some restraint or think about more efficient filtration. Try hand-feeding your fishes for a while to monitor the quantity of food.

Ammonia Levels

In the case of high ammonia levels—and ammonia is highly toxic to fishes, with some simply being more hardy, but none immune to the effect—reducing the ammonia level with water changes and the use of zeolite in the filter will help tremendously. The signs of ammonia poisoning are flashing (rubbing the irritated body and gills on surfaces in the aquarium) and rapid breathing in the initial stages, and as the ammonia levels increase, lethargy, loss of appetite, laying on the bottom with clamped fins, or gasping at the water surface if the gills have been affected.

At high levels (>0.1 mg/liter NH3) even relatively short exposures can lead to skin, eye, and gill damage. An immediate water change is called for. Adding an airstone and one teaspoon of salt per gallon of water, provided there are no delicate plants in the aquarium, will work as an emergency stopgap until you are able to change enough water to reduce the nitrite to a safe level. If water changes have been neglected for some time, and conditions are dreary in the aquarium, it is better to use a combination of water changes and ammonia, nitrate, and phosphate resins plus carbon plus biofiltration to bring the tank back to a healthful state rather than to rely on huge, potentially shocking, water changes with raw tapwater.


Ich, velvet, and skin and gill flukes are among the more common parasites that live on the outside of the fishes. These enemies can be treated with good success provided they are caught early and treated appropriately. Ich and velvet both show up as spots, with ich being white and velvet looking like golden dust. With both diseases, the fish flash frequently and seem to be in acute discomfort. Often, by the time you notice the spots, the parasites are already quite heavy on the fish. The few spots on day one may have increased to a heavy coat by day three. Also, there will be parasites on the gills as well, and the delicate gill filaments you cannot see are often in bad shape while just a few external spots are apparent on the tail and fins of the fish. For virtually any parasite, you are treating both fish and tank. Even if you use a hospital tank, you must treat the evacuated show tank as well. Start with a major water change and gravel vacuuming to reduce the parasite population, then raise the temperature slowly to 88 to 90°F (31 to 32°C). After three weeks, all the parasites will have hatched and died off in the empty aquarium. Bring the temperature back down to normal, then introduce one test fish and observe.

Often, in addition to heat and a water change, the only other medication required is one teaspoon per gallon of salt. Besides stimulating slime coat production, the salt kills the free-swimming parasites, which will stop the infestation in its tracks, provided the treatment is carried out for the necessary length of time: 21 days. If further medication is needed, malachite green and formalin combinations are very effective.

For velvet infestations, the same treatment of hygiene/aeration/heat/salt as above is highly recommended, but if medication is required, acriflavine, used according to manufacturer’s instructions will clear up any residual infestation.

Gill and skin flukes present with similar symptoms of labored breathing and flashing, and breeders will find that their fry will die off a few a days after hovering at the waterline gasping for air. Again, hygiene is a huge part of the treatment; as with any outbreak of disease, start with the water changes. Heavy fluke infestations affecting more than one fish are usually caused by overcrowding, poor water quality, or water with a high organic content. Under these ideal circumstances for pests, parasites multiply rapidly. Again, the hygiene and salt bath regimen is exceptionally valuable. Malachite green and formalin, used according to manufacturer’s directions, will generally help cope with heavy infestations.

Bacterial Infections

Bacteria are everywhere, the good ones, the bad, and the indifferent. Our problems with bacteria in the aquarium are virtually always associated with overfeeding, overcrowding, and poor water quality. Some bacteria show up in the form of fin and tail rot. Other times, the fish’s eyes will bulge from their sockets, and sometimes the bellies of the fish will swell until the scales pop out like pinecones. Some of these bacterial conditions can be treated, some not. Often, the odds are only as good as a roll of the dice, but it is very satisfying when a fish that was very sick recovers and goes on to a long life in the aquarium. Fin and tail rot, open wounds, fungus, pop-eye, dropsy, and many other similar infections do need antibiotics. As it is difficult for most of us to determine which medication is ideal for which disease, manufacturers offer broad-spectrum antibiotics and indicate on the packaging which symptoms the medication is useful against. Consult the packaging for similarity of symptoms. Also, aquarium pharmaceutical companies generally provide support numbers. Avail yourself of the advice of the experienced.

Don’t let fish diseases get you down. Sometimes fish die for no apparent reason. Take heed, and quarantine!

Excerpt from: 101 Best Tropical Fishes
Chapter by: Mary E. Sweeney

See Also: 7 Ways to Reduce Stress in Your Aquarium