Osmoregulation in freshwater fish

  1. I'm interested in understanding this process (Osmoregulation) better. There is some debate about treating freshwater fish by adding medication to the water, and for some instances adding epsom salt.

    I have read that osmoregulation in fish only allows water to pass through the skin, and not minerals.


    Other articles mention how increased magnesium in the water helps the fish draw in more water thus flushing out the system.


    I'm a bit confused as hardness of the water is said to regulate the amount of water a fish absorbs through osmosis, as water moves from an area lower mineral content to an area of higher mineral content. Adding magnesium sulfate to the water would raise GH so how does this relate to increasing the fishes intake of water?

    So when using medications in the water, are these medications absorbed by the fish through osmosis, and how can epsom salt aid in flushing parasites from the intestines when it is added to the water.
  2. jcsd
  3. jim mcnamara

    jim mcnamara 1,466
    Science Advisor
    Gold Member

    This is a good, well written question. There is something you are overlooking. For example, anadromous fishes like salmon and some herrings, survive well in fresh and in salt water, so osmoregulation there is clearly different.

    So, what I am saying is you probably want to focus on a small clade of fishes - a group of closely related fish. What is reported for salmon will not match cichlids, clearly.

    Are you trying to work out something for your aquarium community or are you trying to generalize across fish (which has lots of problems)?

    Adding Magnesium Sulfate (epsom salts) to aquarium water for parasite removal is not necessarily a universal treatment. And the salt may be doing something unpleasant to the parasite rather than changing fish physiology. Aquaria have fish in "unnatural" situations, and frequently at densities/unit volume of water that seldom happen for the same fish out in the wild. So density-dependent parasites flourish.
    Last edited: Mar 14, 2013
  4. I'm basically concerned with african cichlids, and more specifically those from lake malawi. This question stems from a debate on the Cichlid-Forum. It is widely recommended there and other places that specialize in these cichlids that adding epsom salt to the aquarium water will help flush intestinal parasites from the fish which cause "Malawi Bloat". I read a few articles that studied the effects of epsom salt on these parasites when given to the fish orally that found that a magnesium sulfate solution of 7% killed them. That is a much higher percent than we would ever add to the water though, and myself I don't like to add any salts.

    So the question is how can these salts added to the water have an effect on internal parasites? Is it true that only water can pass through the skin and not minerals as stated in the first quote above? How does adding epsom salt to the water result in flushing out the fishes system?

    Also with regard to medication in general added directly to the aquarium water. I understand how these medications can treat external parasites, or bacteria/fungus issues on the fish, but can they treat internal issues? These fish don't drink water, although I imagine that some water enters the system when they swallow food. So is it possible that osmosis draws in medications from the water, and if so would it only be liquid forms of medication, or can minerals (solids) also pass into the fish through osmosis.
  5. I am not sure here what you mean by "internal parasites. If these parasites cause bloating, then they probably live in the digestive cavity of the fish. Malawi Bloat is probably caused by a digestive tract parasite, not a parasite embedded in tissue.

    The digestive cavity is not a tissue. It is open to the outside. The membranes that line the cavity are probably semipermeable, allowing through nutrients and some salts. At least, that is what the membranes that line Homo sapien intestines do.

    Terrestrial vertebrates absorb salts through their intestinal and stomach linings. I conjecture that fish do the same. So maybe they absorb minerals in the water through their digestive membranes.

    My understanding is that fish can absorb nutrients mostly through their digestive tract, the same as we do. Some nutrients may be absorbed through the gills. Some shark embryos absorb nutrients in their amniotic-like fluid through their gills. However, not much can pass through fish scales.

    Fish scales have real bone in them. Very little diffuses through bone. So the scaly fish can only absorb minerals and medicines through the their digestive tract and maybe their gills.

    I suspect that water soluble minerals and water soluble chemicals mostly get absorbed through the digestive tract. Water can also be drawn out of the body fluids of the fish through the digestive tract.

    If you add epsom salts to the water, and the fish swallows it, water may be drawn out of the blood into its digestive cavity. Similarly, epsom salts may draw water by osmosis out of the gill membranes.

    Why can't they drink water? The same swallowing motions that get food into their gut also will get water into their gut. If they keep their mouths closed, water can't enter their gut. I know that some fish swallow air and get air in their gut. Fish with "open air bladders" swallow air to fill their air bladders, and spit out air to empty the bladders.

    According to what I have seen, I think that medicines intended for internal tissues are generally administered through their food or by injection. Their are a lot of parasites that work on the surface of the fish and in their guts. So maybe there are a lot of medicines that are placed in the water for these parasites.

    I worked in an aquarium. The sharks were fed vitamins that were placed in their food. Sometimes, they wouldn't eat the food because it had too many vitamins. The vitamins were not dissolved in the water.

    One puffer fish had an eye infection. The fish was taken out of the water to get its eye drops. The antibiotic wasn't dissolved in the water.
  6. The parasites "Hexamita" are thought to always be present in the intestines. When fish are under stress, or their diet contains hard to digest proteins, the parasites can multiply create blockages, and puncture the intestine linings. They can then enter the blood stream and internal organs causing death in most cases.

    I didn't say they can't drink water, just that they don't. They have no need for it as their bodies are constantly absorbing water through osmosis, and their kidneys are constantly flushing out excess water to keep things in balance. I conceded that some water will enter the stomach when they swallow food, but the epsom salt concentration in the few drops of water they consume this way would be inconsequential.

    Also african cichlids affected by bloat typically stop eating, and it is at this point that people will start to treat the infection. Treatment with Metronidazole or Clout seems to be standard, but these are just added to the water. People will also treat food with Metronidazole for the other fish in the tank that are still eating as a preventative measure as it is thought that the parasites ejected from the anus, along with a clear stringy substance thought to be the mucus lining of the intestines, pass to the other fish when they mouth it testing for food.

    So I'm still no closer to understanding how epsom salt in the water helps with bloat, though this comment

    seems more inline with my thinking that the epsom salt reduces the intake of water through osmosis.
  7. So it seems to me the host needs a good laxative. If the Heximita are in the fish intestines, then the best place for them is in fish poop. Epsom salts are a laxative.

    They may need minerals in the water. There skin probably doesn't absorb salts. The water maybe full of minerals, but the fish can't access them except through the digestive tract. They may "drink" to absorb minerals.

    So these medicines cause the parasites to leave in the fish poop. Mucous is also forced out. Seems to me that these medicines are acting like a laxative.

    I don't understand where the osmoregulation comes in. The article does not say that the fish is loosing salt. It says that the medicine causes it to poop.

    Maybe the fish looses water by pooping. Maybe there are salts that come out of the anus with the mucous. Thus, pooping could interfere with the natural osmoregulation of the fish. However, the parasites are more dangerous than the interference with salt intake.

    I interpreted your statements about “osmotic pressure” and “diffusion” as referring to water transport through the skin. Of course, I agree that some water can diffuse through fish skin. This would especially be true for those fish that don’t have scales. However, some water can also pass through the skin of fish with scales. I agree that the skin participates to some degree with osmoregulation. Fish don’t have sweat glands. Obviously, the kidneys must play a major role in osmoregulation.

    Maybe you think that all salts can pass through the skin of a fish. I don’t think salts pass freely through the skin of a fish. Maybe some water can pass through the skin of a fish. However, the salts probably have a harder time. The salt concentration in the blood of a fish is usually different from the salt concentration in the ambient water. Osmotic pressure can change the concentration of salt in the body only through the transport of water.

    I also interpreted your statements about epsom salts as claiming that their main effect involves osmotic pressure. I disagree with that. As you pointed out, the osmotic pressure associated with the small amounts added to aquarium water is minor.

    The exact composition of the salt is unimportant so far as osmotic pressure is unimportant. One micromolar concentration of a binary epsom salt will have the same osmotic pressure as one micromolar NaCl. Therefore, any medicinal effect that is specific to epsom salts can not be closely associated with osmotic pressure.

    The article that you sited claimed that epsom salts have their medical effect in fish for the same reason that they have medical effects in human beings. For one thing, epsom salts are a laxative. I conjecture this is the main benefit.

    How do the epson salts get in the digestive tract of the fish? I suggest that it is mainly by swallowing. When they swallow food, they also swallow water. Maybe they swallow water also to absorb minerals from the water. Sea water contains all sorts of minerals. I don’t think all those minerals can get in through the skin. So it makes sense to me that a fish would try to balance its diet by swallowing water. If a marine fish needs more potassium, then it may need to swallow water. If a fresh water fish needs to decrease the amount of salt, maybe it can spit out salt water from its gut. In that sense, the digestive tract probably aids osmoregulation. Epsom salts may have a secondary effect on osmoregulation because of its effects on the gut.

    As a “value added benefit”, good digestion probably helps the fishes osmoregulation. I think that is what I was trying to say. I was thinking of diseases in humans such as typhoid fever, where the loss of water through the digestive tract can kill the victim. However, my analogy between human beings and fish may be over extended.

    There may be a secondary effect of epsom salts on the osmoregulation in fish. However, their main effect is most likely on the digestive tract of the fish.

    The article that you cited appears to say that the epsom salts work through the digestive tract of the fish rather than its skin. This seems reasonable to me.

    Perhaps we are both right but with different fish. Marine fish have different problems than fresh water fish.

    Here is an article that claims that the fish kidneys are doing the opposite of what you are claiming. Metabolic induced toxics are diluted in water. However, this is for eliminating the toxins safely. The system works to minimize renal water loss.
    “At low rates of glomerular filtration in seawater fish, NaCl-coupled water secretion serves to increase the renal excretory capacity by increasing the luminal volume into which waste, excess, and toxic solutes can be secreted. The reabsorption of NaCl and water in the distal nephron and urinary bladder concentrates unwanted solutes for
    excretion while minimizing renal water loss.”
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