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11.5. Hollow Belly

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Author : David Bogert

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Time To Read :
5 minutes
11.5. Hollow Belly or sunken belly in tropical fish
11.5. Hollow Belly or sunken belly in tropical fish

If most of the fish in an aquarium have “hollow belly” this is generally due to lack of enough food. If only a few fish have a “hollow belly” the number of possible pathogens is in the hundreds.

Hollow Belly on a Fish
Hollow Belly on a Fish
Causes of “Hollow Belly”:

Fish get “hollow belly” because some pathogen is interfering in the normal absorption of nutrients. Necropsies on dead fish with “hollow belly” show the following incidence:

  • 40% have fish TB (treat with over-filtration and UV)
  • 15% have “hexamita” (treat with metronidazole)
  • 10% have bacterial infections (treat with antibiotics)
  • 5% have roundworms (treat with Fenbendazole)
  • 30% have no clear cause

The biggest cause of “hollow belly” in a well-fed aquarium is fish TB (which scientists call Environmental Mycobacteriosis or “EM”), a persistent and extremely common bacteria found naturally in most tanks. It goes by the name”wasting disease”. There are no medicines for fish TB. Here are some articles on fish tuberculosis:

10.4. Fish Tuberculosis

10.4.1. Fish Tuberculosis in Depth


Fish TB
Fish TB
Treating “Hollow Belly”

“Hollow belly” in a well-fed aquarium should be treated by add large amounts of biofiltration and a UV unit to the aquarium if one doesn’t already have that. These actions reduce the bacterial count in the water which in turn reduces the load on the immune system of the fish. The immune system is thus better able to attack whatever is affecting the fish. Most of the time hollow belly with no associated symptoms is fish TB (environmental mycobacteriosis). Fish TB can be reversed with lots of biofiltration and very clean bacteria free water.

If the fish only has hollow belly as a symptom, clean, bacteria free water is the ONLY treatment which should be given. But sometimes there are other symptoms which show up that indicate an intestinal infection:

  • Stringy white poop
  • Not eating
  • Food spitting

If one of these symptoms shows up in combination with hollow belly the probabilities change with regard to the cause of the hollow belly. Necropsies on dead fish with hollow belly combined with any ot the above three symptoms show the following incidence:

  • 40% have “hexamita” (treat with metronidazole and Epsom salts)
  • 30% have capillaria (treat with Fenbendazole or Levamisole)
  • 20% have bacterial infections (treat with antibiotic)
  • 10% have no apparent pathogen

It is best to isolate the fish in a bare bottom hospital tank and add medicated food. First buy SeaChem MetroPlex and Maracyn 2 (or the optional medication listed below) over the internet.

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.

Feed it to your fish for one month.

The treatment for “hollow belly” has some options:

  • metronidazole, alias metro’ (API General Cure, SeaChem MetroPlex simply mixed into the food, or Hikari Metro Plus food or New Life Spectrum Hex-Shield food)
  • Broad spectrum antibiotic (Midland Vet Service Aqua-Mox, VetDepot Amoxicillin, Fishbiotic Ampicillin, Mardel Maracyn 2, SeaChem KanaPlex)

Also note the directions with most aquarium medications tell one to put large amounts of the medication into the aquarium’s water. This is a huge scam perpetuated by suppliers of aquarium medications. Fish don’t drink! And they also don’t absorb medications through their skin or gills. This controversial topic is covered in this link:

12.5. Fish Don’t Drink

Also note than when presented with hollow belly many people start feeding their fish more. Starvation is rarely the cause of hollow belly, especially if the symptoms are in only a few fish. Of course if all fish have hollow belly look hard at the amount of food. But if one is feeding an amount of food equal to two eyeballs in volume (six fish = twelve eyeballs) once a day added feeding may only make the problem worse.

hollow belly or sunken belly
hollow belly or sunken belly

Here is an article on how to calculate how much to feed:

3.3. How much to Feed Fish

The symptom of hollow belly is often accompanied by another symptom. These symptoms are covered separately in these articles:

11.2. White Poop

11.15. Fish not eating


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