Dealing with sick fish is hard and often overlooked. It can be difficult to know when a fish is sick and even harder to identify if it’s a parasite, infection, or something else.

From there, you need to determine if the parasite is internal or external or if the infection is bacterial or fungal. It’s a lot of work to make a correct diagnosis and start treatment, and unfortunately, many hobbyists catch these diseases too late.

The most important step to treating a sick fish is being prepared. Many of these illnesses pop up overnight, leaving a healthy-looking pet fish gasping for air at the bottom of the tank the following day. It is at times like these that a quarantine system and an assortment of medications can make treatment quick and effective.

Some of these medications are antibiotics meant to combat bacterial infections. But how do antibiotics for pet fish work in the aquarium setting, how do you know when to start fish antibiotics, and what are the best medications available? Today’s post is all about answering these questions. Let’s start with the first…

How Do Fish Antibiotics Work?

To understand when to treat with antibiotics, we need to first understand how fish antibiotics work.

It’s important to realize that fish antibiotics do not magically cure fish of their ailments. Instead, fish antibiotics decrease the growth of bacteria populations until the immune system of the fish is able to recover and naturally build resistance.

Simply put, fish antibiotics interfere with the physiological and reproductive abilities of bacteria, slowing or stopping growth and spread. In the aquarium setting, there are two main types of bacterial infections you need to be aware of: gram-positive bacteria and gram-negative bacteria.

Gram-Positive Bacteria vs. Gram-Negative Bacteria

Differentiating between bacteria can be complicated, but it’s necessary for accurate diagnosis and treatment. In short, the difference between gram-positive bacteria and gram-negative bacteria is their cell wall composition.

  • Gram-positive bacteria have a thin cell wall surrounded by an outer membrane. Gram-positive bacterial infections are less common in the aquarium setting and usually consist of Streptococcus spp..
  • Gram-negative bacteria lack an outer membrane but have thick polymer layers instead. Most aquatic bacterial infections arise from gram-negative bacteria, like Aeromonas spp., Flavobacterium spp., Vibrio spp., and Pseudomonas spp..

The most definitive way to tell gram-positive bacteria apart from gram-negative bacteria is by performing a gram stain, hence the name. Gram-positive bacteria will stain blue while gram-negative bacteria will stain pink.

As most hobbyists aren’t able to perform such a test, only physical symptoms can be observed for diagnosis. Gram-negative infections are typically more aggressive and deadly to fish.

Identifying Bacterial Infections

A bacterial infection can be difficult to diagnose. They often happen alongside other diseases and infections with similar symptoms, especially fungal infections. However, bacterial infections are much more common than true fungal infections as bacteria readily live in aquarium water, waiting to affect a compromised pet fish.

The most common symptoms of a bacterial infection are:

  • Inflammation or redness
  • Irregular mucus/slime coat
  • Fragile and frayed fins
  • Cloudy or swollen eyes
  • Bloating

Infection is largely caused by injury alongside poor water conditions and/or malnourishment. It is possible for your fish to fight off infection if water quality is good and diet is maintained, though sometimes even the strongest pet fish can succumb to harmful bacteria.

It is near impossible to differentiate between gram-positive bacteria and gram-negative bacteria without a gram stain. Since gram-negative bacterial infections are much more common (in freshwater), most hobbyists use a fish medication that will target those bacteria first. In marine fish, gram-negative is more common.

How Do Fish Antibiotics Work?

To understand when to treat with antibiotics, we need to first understand how fish antibiotics work.

It’s important to realize that fish antibiotics do not magically cure fish of their ailments. Instead, fish antibiotics decrease the growth of bacteria populations until the immune system of the fish is able to recover and naturally build resistance.

Simply put, fish antibiotics interfere with the physiological and reproductive abilities of bacteria, slowing or stopping growth and spread. In the aquarium setting, there are two main types of bacterial infections you need to be aware of: gram-positive bacteria and gram-negative bacteria.

Treating Aquarium Fish With Antibiotics

Knowing what to treat is equally as important as correctly treating. Fish antibiotics have exact directions and the treatment schedule needs to be followed carefully.

How do these antibiotics move from the water into your fish, though?

How To Give To Your Pets

The best way to give fish antibiotics is through treated food. This lets the medicine be administered in more direct, higher doses and saves some damage to nitrifying bacteria. A problem arises when your pet fish refuses to eat though, making this method impossible.

Most fish antibiotics can be dosed directly into the aquarium. From there, these medicines need to be incorporated into the body of the fish, but how? This involves some physiology.

Fish experience osmoregulation1. Osmosis is the process of a solvent moving from an area of low concentration to an area of high concentration across a semipermeable membrane in order to create a balance between the internal and external environments. Osmoregulation differs between freshwater and saltwater life. Here’s a video from Its AumSum Time. The explains the difference between freshwater and saltwater fish.

Freshwater fish are hypertonic, meaning that their internal environment has more salt than their external environment. Because of this, water flows in through the gills and is absorbed by the body.

On the other hand, saltwater fish are hypotonic, meaning that their internal environment has less salt than their external environment. Because of this, water wants to leave their body. In order to compensate for this loss, saltwater fish need to actively drink water and conserve it by sending some to their digestive tract.

In the aquarium setting, this is an advantage to saltwater species. Because saltwater fish actively take water into their bodies, soluble antibiotics are much more likely to immediately enter the internal bloodstream in the marine environment. Still, freshwater fish can be given soluble antibiotics, though feeding them is much more effective.

In general, there are two ways to administer fish antibiotics for both freshwater and saltwater fish. If your fish is suffering from an external infection, then bath treatments are recommended, though they can sometimes be ineffective. If your fish is showing signs of internal infection, then food treatment is the best route.

Bath Treatments

Bath treatments are the preferred choice of treatment for external infections when the fish is refusing to eat or there are other limitations.

Bath treatments are best when there is no access to a quarantine system that allows for fish antibiotics to be directly dosed into the aquarium. This could be due to sensitive corals and invertebrates or where there is large biological filtration that could be affected.

The problem with bath treatments is that very little medicine actually enters the bloodstream of the fish. In addition, the constant transferring of the fish can be stressful and cause injury; if the fish dies during this transfer, then there was probably little hope for it in the first place.

Antibiotic-Treated Food

The best treatment for bacterial infections is through food mixed with fish antibiotics. This requires a binding factor, like Seachem Focus, so that the fish antibiotics do not leak out of the food.

This method allows the medication to be administered in large doses. However, it requires that the fish is still actively eating, which means that the infection has not yet compromised the fish to a large extent. Still, appetite can sometimes be increased through garlic or live food methods.


Lastly, injection is a possible course of treatment, though many hobbyists do not have this option available to them.

An injection is the best and most effective method of treating infections in large and expensive fish, like oscars or koi, but is often unattainable for the average hobbyist.

The 9 Best

1. Cephalexin 

Thomas Labs Cephalexin offers two grades of fish antibiotics: Thomas Labs Fish Flex and Fish Flex Forte. Though this antibiotic is labeled as ornamental fish antibiotics, hobbyists have actually found it to be most effective against invertebrate bacterial infections, especially those found in anemones.

This is a very broad fish antibiotics and needs to be administered in a quarantine system or bath for 5-10 days.

2. Ciprofloxacin

Again, Thomas Labs offers two grades of this antibiotic: Thomas Labs Fish Flox and Fish Flox Forte.

Ciprofloxacin is much more effective as a fish antibiotic than cephalexin, though this product is regularly used to treat anemones and other invertebrates as well. Specifically, this medicine can be dosed in a bath or in a quarantine tank for gram-negative bacterial infections over the course of 5-7 days.

3. Amoxicillin 

Amoxicillin is a very popular antibiotic for other animals and human use. However, amoxicillin isn’t commonly used in the aquarium as a viable course of fish antibiotics. Regardless, Thomas Labs carries Thomas Laboratories Amoxicillin Fish Mox and Fish Mox Forte.

If for anything, amoxicillin is one of the few fish antibiotics, alongside penicillin and ampicillin, used for treating gram-positive bacterial infections. These fish antibiotics should not be used with invertebrates and should be administered in a quarantine tank or bath treatment for 5 days. Still, hobbyists prefer other fish antibiotics first.

Can You Use Amoxicillin In The Aquarium?

Never use antibiotics intended for human consumption in the aquarium and vice versa. Medications should always be dosed seriously and carefully. Aquarium brands have specific formulas for making ornamental fish drugs that are not for human use.

While aquarium brand amoxicillin can be used in the aquarium, hobbyists usually prefer other options first. Amoxicillin is very popular for treating other animals and humans but is generally useless in the aquarium setting. One of the only ailments hobbyists have found amoxicillin to be useful for is treating fungal infections in the eyes.

5. Seachem KanaPlex + Seachem MetroPlex + API Furan-2

Seachem KanaPlex, MetroPlex, and API Furan-2 are very common fish antibiotics to dose together. These three medications can be incredibly effective at combatting a variety of infections.

Seachem Kanaplex is a 7-day kanamycin-based medicine recommended for:

  • Popeye
  • Cloudy eye
  • Fin rot
  • Dropsy
  • Mouth rot
  • Hemorraghic septicemia(Aeromonas hydrophilia)

MetroPlex should be dosed at the same time, though can be administered up to 3 weeks on its own. This antibiotic is made from metronidazole and is meant to treat protozoan parasites and anaerobic bacterial diseases (Cryptocaryon, Hexamita, Ichthyophthirius). Mainly, MetroPlex is used for ich, hole in the head/head and lateral line erosion (Hexamita spp./Spironucleus vortens), and velvet.

API Furan-2 targets both gram-positive and gram-negative bacteria through a 7-day nitrofurazone-based treatment. This antibiotic specifically treats for:

  • Bacterial gill disease
  • Open red sores (Aeromonas spp.)
  • Body slime
  • Cloudy eye
  • Columnaris (Flavobacterium columnaris)
  • Fin rot

This combination of fish medications can cover a wide range of infections as long as the treatment is carried out in full.

6. Seachem SulfaPlex + NeoPlex

Seachem SulfpaPlex and NeoPlex can be used together at the same time or separately depending on the illness.

SulfaPlex is a general sulfathiazole-based antibiotic meant to treat bacterial, fungal, and protozoan-related illnesses. Though this product can be used in both freshwater and saltwater settings, it is most effective to travel across gills in saltwater.

Specifically, SulfaPlex can be used to treat:

  • Fin rot
  • Hemorraghic septicemia(Aeromonas hydrophilia)
  • Fur coat syndrome
  • Mouth rot
  • Fungus

NeoPlex is another broad neomycin sulfate-based treatment for external infections, like fin rot, bacterial lesions, bloat, and mouth rot. Most hobbyists describe it as having the same effects as topical Neosporin.

Both of these medicines should be used for at least 7 days but can be dosed for up to 3 weeks on their own.

7. API Triple Sulfa

API Triple Sulfa is one of the safer fish antibiotics treatments to use in the aquarium as it won’t completely kill all nitrifying bacteria. Still, filter media should be removed before dosing for best results.

Sulfa medications are thought to be somewhat outdated. Many hobbyists have found some resistance to these treatments and opt for stronger alternatives, like Furan-2. However, API Triple Sulfa has the benefit of being gentle on the display tank ecosystem if need be.

API Triple Sulfa can treat gram-negative bacteria, like those that cause:

  • Hemorrhagic septicemia (Aeromonas hydrophilia)
  • Bacterial gill disease
  • Fin rot
  • Cottonmouth diseases
  • Body slime
  • Cloudy Eyes

Treatment should last 10 days.

8. Enrofloxacin

Enrofloxacin is one of the most modern ways to treat bacterial infections in fish2, though it is still best administered in a quarantine tank or through 5-hour baths. Though effective, this medicine can be difficult to find; it is most commonly found as Baytril from Bayer Pharmaceuticals.

This is a fast-acting fluoroquinolone antibiotic that can help treat gram-negative and gram-positive infections.

9. AAP Spectrogram

As of 2021, AAP Spectrogram has become difficult to find fish antibiotics; this is likely in direct response to heightened pharmaceutical regulations during the Covid-19 pandemic.

If you come across AAP Spectrogram, it can be very useful for treating both gram-negative and gram-positive bacterial infections. This fish medicine is a combination of KanaPlex and Furan-2 (kanamycin/nitrofurazone), but much easier to use and more effective. It can be used to treat most infections besides parasitic ones.

Which Are Safe?

All medications that are labeled for aquarium use are safe for the aquarium. However, it is possible to overdose on some medications so it’s important to follow directions closely.

There is one thing you should avoid though, and that’s ‘natural’ medications.

Avoiding ‘Natural’ Antibiotic Forms And Medications

While many reputable aquarium brands offer great products to their consumers, some brands take advantage of branding treatments. Some of the latest trends in the aquarium hobby have arisen from ‘natural’ fish antibiotics and medications which end up hurting fish more than helping them. Some of these organic alternatives include tannins and natural oils.

Tannins can be very beneficial to aquatic systems and can lead to healthy fish, however, they don’t exactly help with bacterial infections. Research has found that tannins are only effective at combatting bacteria at very high concentrations3, much more than is reasonable for any tank size. Tannic acid is also a blood coagulant with can decrease healing rates.

Natural oils, like those found in MelaFix and PimaFix, are also detrimental to the health of your fish. Though these oils might look and smell medicinal, they have been linked to swim bladder damage and have been found to be nearly ineffective at low concentrations. Similarly, aloe vera also inhibits your fish’s ability to exchange oxygen and increases available sugars for bacteria to feed on.

Though it might seem like a good idea to stay as natural as possible when it comes to treating fish diseases, it is usually safer and easier to use tried and true fish antibiotics.

Can You Give Your Fish Over-The-Counter Human Types?

Fish should never be given medications intended for human consumption. Though the chemical makeup may be similar between medicines, factors like dosages, solubilities, and efficacy cannot be known.

It is true that some fish antibiotics have become difficult to find recently. Though you may not be able to get the antibiotics of your choice, there are many alternatives for each treatment from reputable aquariums brands like API and Seachem. This makes treating your fish safe, fast, and easy.

Can Humans Take Fish Meds?

Also no! You should never be using an antibiotic you haven’t been personally prescribed. Not only does this put your own health at risk, but it can also make it difficult for other hobbyists to obtain these fish antibiotics in the future, like AAP Spectrogram.

In case of a medical emergency, consult your primary doctor.


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