Red Mouth in Goldfish

10.3.10. Red Mouth in Goldfish

There is a bacterial disease that causes a red mouth in goldfish called enteric red mouth disease. This bacterial infection can cause subcutaneous hemorrhage that presents as reddening of the throat, mouth, gill tips, and fins, and eventual erosion of the jaw and palate. Hemorrhaging also occurs on internal organs, and in the later stages of the disease, the abdomen becomes filled with a yellow fluid – giving the fish a “pot-bellied” appearance. The fish often demonstrate abnormal behavior and stop eating.

The causative agent of enteric red mouth disease is the bacterium Yersinia ruckeri. This disease is most common in pond fish and rare in the aquarium. It is occasionally seen in goldfish.

Outbreaks usually begin with low mortalities that slowly escalate. The severity of the outbreak depends on the strain and presence of stressors. It is treated the same as bacterial hemorrhagic septicemia as the Yersinia ruckeri bacteria is a gram-negative bacteria.

red mouth disease (could also be herpes)
red mouth disease (could also be herpes)


Red Mouth should be treated with broad-spectrum antibiotics (Midland Vet Service Aqua-Mox, VetDepot Amoxicillin, Fishbiotic Ampicillin, Mardel Maracyn 2, SeaChem KanaPlex, API Fin And Body Cure).

These medications are only available over the Internet. Fish stores have all gone over to “natural” medications which have a very high profit margin and simply put, don’t work.

Bacterial infections are most effectively treated with antibiotics in the food. Many believe (and the instructions on the antibiotics say!) that antibiotics need to be added to the water. They are simply incorrect. This controversial topic is covered in the following link:

12.5. Fish Don’t Drink

Red mouth disease in a fish
Red mouth disease in a fish

It is easy to make medicated food. Heat 1/4 cup water (two ounces or 58 milliliters, not a lot) in the microwave. Then blend seven grams of plain animal-derived gelatin (Knox gelatin, one packet) into the hot solution with vigorous stirring. Take two tablespoons of dry commercial fish food (pellets or flakes) and mix it with just a little of the hot water/ gelatin mixture. Add hot water/gelatin until you get a paste-like consistency. If it gets too watery just add more food. To get gel food to float simply mix in a little whipped cream at this point.

Then add just a “smidgen” (roughly 1/16 teaspoon, a 1% to 2% addition) of medication to the mud. If you are using more than one medication mix the medications together, then use just a “smidgen” of the mixture. If you are using a packet of medication, take just a “smidgen” of the packet contents. Mix and mash the whole mass thoroughly.

All the fish in the aquarium should be fed a steady diet of antibiotic-laced food for at least ten days. Note that the exact amount of medication that goes into the food is not very important. Antibiotics can be overdosed pretty much with abandon as they are only toxic in large doses over a period of months. Note antibiotics in the food do not affect the filters so they can be left in place and operating.

If you can’t resist the urge to treat the water, remove the biofiltration media (including sponge and/or foam) in the filters and put it in an open container for the duration of the treatment. Sometimes antibiotics kill the beneficial bacteria and sometimes they don’t. In any case, the filter media will denature the antibiotics. Monitor the ammonia and would do a 50% water change if it spikes above 1 ppm. Reduce the amount of food fed by 2/3 rds.

Note that if antibiotics are not available, it is quite easy to take a pill or capsule of human antibiotic and use it for fish. If it is a pill just grind it up. Just be aware that human antibiotics are about ten times more potent than aquarium antibiotics, so just a “smidgen” in the food is more than enough. This is a very good option for the folks in Europe or Canada, where fish antibiotics are illegal.

Red Mouth Disease
Red Mouth Disease

The “Nuclear Option”

Any internal bacterial infection that has become advanced enough to cause red mouth in goldfish is a very advanced bacterial infection. The mortality rate is going to be very high no matter what the treatment. It is always difficult to tell a fish keeper their fish are probably going to die but with red mouth that is just the “cold hard facts”.

Many experienced fish keepers use a “nuclear option” to treat red mouth, with three treatments done simultaneously:

  • antibiotics in the food,
  • sulfa drugs and/or furans as a one hour baths
  • sulfa drugs and/or furans in the aquarium water.

There is only anecdotal evidence that this works but if one is so inclined one can do it.

We cover antibiotics in the food above. Here is how to use the sulfa drugs or furans.

Sulfaguanidine, sulfathiazole, sulfamethazine and sulfacetamide are all that are called “sulfa drugs”. Mardel Maracyn Plus is a combination of two anti-bacterial drugs, sulfaguanidine and trimethoprim (trimethoprim  is an antibiotic with an action that is very similar to furans). SeaChem SulfaPlex (Sulfathiazole) is a sulfa drug. API Triple Sulfa (Sulfathiazole, Sulfamethazine and Sulfacetamide) is a sulfa combination.

“Furans” include nitrofurantoin, nitrofurazone and furazolidone. Bifuran is nitrofurazone and furazolidone. SeaChem Focus is “polymer-bound” nitrofurantoin. Sulfa and furans have very similar properties with furans being more toxic to fish than sulfa drugs,

To treat a fish with red mouth, make up a bath with a sizable dose of the furans or sulfa drugs. Use five time the recommended dosage of a furan drug or ten times the recommended dosage of the sulfa drug. Put the fish in the heavily dosed water for one hour and only one hour. Then return the fish to an aquarium. Repeat this once every 24 hours, no more.

Then follow the directions on the package of furans or sulfa drugs as to adding the medication to the aquarium water. This dosage in the aquarium water will not bring up the level of drug in the fish to therapeutic levels but will prevent the bacteria from being transmitted from fish to fish.


If one has more than one fish with a bacterial disease, one must treat the whole aquarium. This is an emergency. Don’t fool around with herbs, tree leaf oils or some ineffective treatment. Ben Ochart treated a bacterial infection with Pimafix and Melafix. They did nothing to stop the infection. He lost a lot of large beautiful fish before he stopped the infection with antibiotics and sulfa drugs. This link covers the snake oil medications such as Melafix and Pimafix:

12.4.1. Natural Medications

The entire topic of bacterial infections in tropical aquarium fish is covered in more detail in this link:

10.3.11. Treating Bacteria

Startpage Aquariumscience

Source: – David Bogert

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