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10.3.11. Treating Bacteria

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Author : David Bogert

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Time To Read :
9 minutes
10.3.11. Treating Bacterial Diseases in Fish
10.3.11. Treating Bacterial Diseases in 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 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.

It is very important to have antibiotic on hand and begin immediate treatment when a bacterial disease is detected. Treat the whole aquarium with a good antibiotic. To do that:

  • For internal bacterial infections like septicemia or popeye add antibiotics to some food and feed half the normal amount of food. Antibiotics for internal bacterial infections are only effective taken in the food.
  • If it is an external infection (i.e. the bacteria have attacked the skin of the fish) also add the antibiotics to the food as the bacteria is invariably also attacking the fish internally and that is what kills the fish. If you want to be more comfortable add the antibiotic to the water but be aware this can be very expensive and does little.
  • If it is an external bacterial infection and you are adding antibiotic to the water, take the biofiltration media out of the filters along with any sponge materials and put it in a bucket of water for the duration of the treatment. The antibiotic will be biodegraded as it goes through the media if you leave the media in place. Sometimes the antibiotic kills the beneficial bacteria and sometimes it doesn’t. There is no way to predict that outcome.
  • Keep measuring the ammonia every 12 hours or so and change the water if it spikes.

Antibiotics kill external and internal bacteria on and in fish and have a definite role in any aquarium. Antibiotics are approved for use in humans, so they are very safe and effective.

Bacterial disease on a fish
Bacterial disease on a fish
Gram Negative and Gram Positive

Bacteria can generally be put in two separate groups, the gram-positive and the gram-negative groups. Antibiotics are of three general groups: broad spectrum (effective against both groups of bacteria), effective against gram positive, and effective against gram negative. The most frequent cause of bacterial infections in fish, columnaris (Flavobacterium columnare), is gram negative.

While columnaris is the most common bacterial pathogen in an aquarium the exact bacteria species can’t really be determined by the hobbyist. Sometimes other bacterial species are at fault in bacterial infections of tropical freshwater aquarium fish. But the best bet is to always treat with a broad spectrum antibiotic which covers gram negative bacteria.

Note that if you have multiple fish dying with no discernable reason, suspect a bacterium and treat with antibiotics. This will give you the best chance of stopping the unknown pathogen.

All antibiotics don’t kill all bacteria. So, treating with antibiotics is a bit of a crap shoot, you can be unlucky and inadvertently chose the wrong antibiotic. But don’t mix antibiotics. Per the University of Florida:

“Combining different antibiotics is generally not recommended. Antibiotics work at many different sites on and in the targeted bacterial cell. Using more than one antibiotic can result in interference between them and, as a worst-case scenario, the antibiotics can essentially ‘cancel each other out.’ Most bacterial infections can be treated effectively with a single antibiotic”

To rid the fish of a bacterial disease, treat the fish with a broad-spectrum antibiotic.

Red Blotch or hemorraghic septicemia
Red Blotch or hemorrhagic septicemia
How to Use Antibiotics

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

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 one 1/4 ounce of plain animal derived gelatin (Knox gelatin, one envelope) into the hot water with vigorous stirring. Take two tablespoons of dry commercial fish food (pellets or flake) 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.

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.  Spread it out into a pancake about 1/8th inch (3 mm) thick on a plastic film or a plate. Then put in the refrigerator. If you plan on keeping it for more than two weeks put it in a small plastic bag and freeze.

Many do not like going to all this trouble and just mix wet commercial food with powdered antibiotic. When this food is put in the aquarium it will rapidly disintegrate, dissolve and cloud the water. The fish typically can eat very little of it. At least bake the food in a low temperature oven till it is dry and like a cracker. This makes the fillers in the commercial food act as a binder which will keep the food together in the aquarium longer.

All the fish in the aquarium should be fed a steady diet of the antibiotic laced food for at least ten days.  Note that the exact amount of medication which 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 this gel food is 80% water. So it will require five eyeballs per day of food per fish (six fish = six fish eyeballs times five).

If the disease continues to progress you might have a gram-positive bacterial infection. Switch to erythromycin (API E.M. Erythromycin, Mardel Maracyn) or penicillin.

Note that if one has large fish such as Oscars or koi one can inject the antibiotics into the fish. This is the best way to administer antibiotics but it takes a steady hand and some knowledge of fish anatomy. It is important to inject the fish in a muscle mass that doesn’t contain any organs or main nerves. This is typically in the upper rear quadrant of the fish. And think very tiny amounts of injection of very tiny amounts of antibiotics.

Septicemia or Red Blotch
Septicemia or Red Blotch
Best Antibiotics for the Aquarium

The best aquarium broad spectrum antibiotics are as follows (with the most effective first):

  • amoxicillin ( Midland Vet Service Aqua-Mox, VetDepot Amoxicillin, Fishbiotic Ampicillin) $10-$20
  • minocycline (Mardel Maracyn 2) $8-$17
  • tetracycline (API T.C. Tetracycline) $8
  • doxycycline (API Fin and Body Cure) $18
  • kanamycin (Seachem Kanaplex) $18

All these antibiotics can now only be obtained over the internet. Most local fish stores have gone over to the completely ineffective but high profit margin “Natural” medications. Note also that with COVID some of these antibiotics have become hard to get and very expensive.

Other Antibiotics

Be aware that some strains of bacteria are becoming resistant to first line antibiotics. So, switch antibiotics if the infection continues in the hospital aquarium.

If on is hunting for a good “cure-all” treatment that won’t break the bank, use oxalinic acid. It’s the poor man’s antibiotic and used in ponds.

Note that metronidazole is often listed as an antibiotic. As an antibiotic it is only useful against some anaerobic organisms such as clostridium difficile (c. diff). These organisms only are secondary invaders in “stringy white poop” disease (some call this “hexamita”). Virtually all pathogenic bacteria are NOT anaerobic and will not be affected by metronidazole. So this isn’t really useful as an “antibiotic” for most fish diseases.

Note also that 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 the human antibiotics are about ten times more potent than the 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.

Note also that one can get a very effective broad spectrum antibiotic called “Baytril” (Enrofloxacin) from veterinarian supply stores (it is used to treat cats and dogs but not humans). Some people in the UK and Canada have been able to get this from the internet from pigeon racing websites.

Fin rot
Fin rot. blackspot and slime coat 
Creating Antibiotic Resistant Bacteria

Several sources recommend against antibiotic treatment as this will supposedly result in antibiotic resistant strains of bacteria. The chances of producing an antibiotic resistant strain of bacteria in a home aquarium are around one in a billion. Commercial fish breeders in southeast Asia use antibiotics by the ton with millions of fish. They probably have already produced several antibiotic resistant strains of bacteria. The chances of some hobbyist doing that are infinitesimally small. Don’t worry about it.

In any case, just think about the idea of not using anything (like an antibiotic) because it might result in a resistant strain of anything (like a resistant bacteria). Any EFFECTIVE medication runs the risk of creating a resistant organism. So IF Melafix was effective against bacterial infections, using it will, in theory, ultimately result in Melafix resistant bacteria. So by this line of logic one should never use Melafix. Ergo the only way to prevent creating a resistant organism is to use an ineffective medication. So the logic becomes use an ineffective medication and let your fish die.

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Return to Diseases Menu

Return to Bacterial Diseases Menu

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