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Update: Blisters On Pearlscales

Guest gFishOnTheBrAiN

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Guest gFishOnTheBrAiN

Hi All!

It's been a looooong time since I posted. I gave up on fishkeeping after losing my two adopted Goldies. But now I?m back in with both feet - saw a big, adorable black moor at a LFS and fell in love all over again.

So, as a glutton for punishment, I want to add a Pearlscale to my aquarium. I and other fishkeepers experienced some weird problems with these guys before - specifically, strange water blisters that appeared seemingly at random. Most resolved/reappeared/resolved on their own with no problems. On occasion, however, some would die from the outbreak. Before launching into ownership again with these fish, I've attempted to research this issue a bit more and collect as much info as I can.

What you will see below is theory/speculation/best-guess. But it may give some people ideas about where to go if this issue recurs for them. I didn't feel it was appropriate to post an "article" as I threatened to do a couple years ago in this previous topic. This is just a list of links, quotes and comments. Quotes are in plain text. My comments are in bold italics.

As always, feel free to add/comment/speculate in any way you think might be helpful!


First - Issues which are caused by infections. This was not the main problem most posters I discussed this with had. Still, the symptoms can be similar when they first manifest, so it's important to know the features of an infection so it can be properly treated.


One or many fish have blisters forming on the skin that are full of a clear to yellowish fluid. The blisters pop and turn into large sores. ---> Furunculosis (Aeromonas Salmonicida)

Even less common - and not experienced at all by the posters I talked to - is Gas-Bubble Disease


Now - on to the seemingly benign (though unsightly and disturbing) blister outbreaks. The blisters most often mentioned in the posts were protruding bubbles of clear liquid, often seeming to originate between the scales of our goldfish. These would break when touched or on their own, leaving little to no visible damage to the fish. Sometimes, an infection took hold if the blister was broken prematurely (though I did not experience this issue). A poster on another board described her theory for the outbreaks her fish were experiencing:


The bubbles seem to start to develop whenever the pH drops to around 7.6. Keeping the pH higher seems to inhibit them forming regardless of other water conditions. The fish seems to do best when the pH is around 8, which is on the high end for goldfish. Any antibiotics or combination I've tried have done little if anything about the bubbles, but they do take care of the secondary infections that are likely when the bubbles pop.

Based on what I've read about osmoregulation and what I've seen on other forums where people have reported this condition after adding aquarium salt, here is what I suspect is happening. Pearlscales may have a tendency to accumulate excess salt in their skin, especially beneath their scales. As pH drops, water accumulates in these areas more rapidly than the fish's systems can remove it, resulting in these bubbles.

pH has not be brought up as often as has the application of salt. Many posters I talked to saw a correlation between the addition of salt to the water and the appearance of blisters. Though seemingly separate, the two parameters (pH and salinity) may point to an osmoregulation issue which could manifest as blisters.


What is Osmoregulation?

Osmoregulation is the technical term for the physiological mechanism in fish that controls the amount of salt and water in their bodily fluids.

As its name implies, osmoregulation is based on osmosis, a process involving the movement of dissolved substances from a stronger concentration through a semi-permeable membrane to a weaker one. The body contents of freshwater fish are saltier than the water they live in, and their skin is semi-permeable. Osmosis will thus cause bodily salts in the fish to ?leak? into the water.

Freshwater fish need to constantly take in water and extract salts from it to replenish what has been lost. This extraction is carried out by special cells in their gills. The constant intake of water means they also have to eliminate it by producing vast amounts of urine (some produce their own body weight in wee every three or four days!)


So how does this affect fish.

In freshwater fish, especially the ones from softwater regions they need to be able to regulate this process or water will invade their bodies...

The surrounding water is constantly trying to invade the fish and dilute its fluids because they contain various salts and minerals. To compensate for this, fish that live in soft fresh water have evolved bodies that quickly get rid of excess water and at the same time hang on to the important salts.

They do this by passing lots of very dilute urine almost constantly. Their kidneys and other organs have evolved to do this over thousands of years. And in their own environment the system works perfectly.

Now just think what happens if such a fish was placed in hard water, or worse still water with some salt added to it. All of a sudden the surrounding water is the stronger solution and the fish the weaker solution. So the fluid within the fish leaves, helped by the fishes own body because of how it has evolved and the fish finds itself having to try to get rid of excess salts but again its body has evolved to hang on to salts not get rid of them. This actually causes the fish to dehydrate.

Obviously this puts the fish under a great deal of stress and eventually it causes some damage, most commonly kidney stones, and ultimately kidney failure once this happens a premature death soon follows. This is what I mean when I say that most freshwater fish do not have the physiology to deal with salt being added to their water.

For fish that live in a salty environment (marine and brackish water) the situation is slightly different. They have evolved in a different way.

Their bodies have evolved to get rid of excess salt and hang on to the water, the main ways they do this is by drinking lots and passing very little but very concentrated urine. Again in their own environment the system works perfectly. But if a brackish water fish is placed in fresh water it still takes in lots of water and its kidneys still produce very little but concentrated urine. The result again is stress on the fish.

Ultimately the osmo-regulation system breaks down and the fish takes in more fluid than it can get rid of (Dropsy). this is one reason why so many Mollies fall victim to this condition when kept in a salt free community tank.

This is why it is very important to keep fish in the proper conditions and why it is not a good idea to routinely add salt to freshwater aquariums.

For additional explanations of osmosis and pictures, see: http://www.fishdoc.net/water/osmoregulation.htm

So, it's clearer how a change in salinity could result in a fish rapidly losing water to its environment - or taking in too much water when its system can no longer keep up with normal osmotic regulation. While osmotic exchange usually happens via the gills and urination, Pearlscales do not have a uniform thickness across their scales. The "pearl" is raised and hard, surrounded by a weaker tissue making up the outside edges of the scale. These weaker areas, and the junctions between them, could be an "Achilles' Heel" when it comes to osmotic pressure changes and rapid water loss. Could blisters also form when a fish takes in too much water? Some have postulated that these blisters could be the first signs of dropsy in a Pearlscale.


Finally, how does pH play into the development of blisters?


Fishes, like most animals, need to maintain critical levels of salts, alkalinity (buffering capacity), and dissolved organic compounds.

Regardless of the method a fish uses to maintain this balance, osmoregulation is usually energy-intensive, affecting growth and swimming performance

Osmoregulation refers to regulating the total ion concentration, whereas ion regulation is the regulation of individual ions in blood and tissues.


...a high density of fishes leads to elevated CO2 levels which in water, becomes carbonic acid (H2CO3).

This lowers the pH of the water and therefore the fish. Fish respond to this by using their metabolically produced bicarbonate ion (HCO3-) to buffer their pH and also excete H+.


Fish maintain pH by adjusting the biocarbonate equilibrium. Short term raising of blood pH can be accomplished by hyperventilation.

Fish also use ion exchange, exchanging H+ for Na + and Cl - for HCO3 -.

However, ion exchange may create one problem (salt imbalance) while attempting to maintain pH balance.

That's all I've found for now.


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  • 5 months later...
  • Regular Member

wow that is great and thank you for the time and indepth posting of that article. One of my pearlies had this and I could not figure out why. She had just been through a round of maracyn and maracyn two for what I thougt was fin rot and the blister where unaffected by the antibiotics as you mentioned. I turned out she really had a tumor which with the area it was located was cutting off the blood supply to her tail and fins. She developed these blisters before she died. I had tried using some salt in the water for the fin rot after the antibiotic treatments. I guess from the gist of your article that is what caused it. Thanks I have salt in my tank right now because one of my pearlies bumped in to something in the tank a halfway broke the very tip end of her tail. I believe I will take it out this next water change

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  • Regular Member

We've seen several cases of this on another board as well.

Here's a clear picture of them on my pearlscale. You can also see the petechiae that showed up at the same time.


We think there is a reasonable chance they are a pre-dropsy symptom, similar to popeye in other varieties, indicating the fish is having trouble with osmoregulation.

I think the pH discussion is a bit of a red herring. Adjusting the hardness of the water (with magnesium [epsom salts] and calcium) and/or the salinity seems to help the pearlscales reduce these blisters. That will not address the underlying cause, however, but can make healing and treatment of the underlying issue easier.

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  • Regular Member

Thanks for posting this, I was wondering about the blisters I read about before--and it makes sense with the osmoregulation and the balancing of solutions in and outside the fish. I've been trying to gather more information on ailments pearlscales seem to suffer over the other goldfish varieties... and it's kind of hard when they are less popular than the usual fish that end up on the disease and emergency forums.

I've heard by several different sources that pearlscales seem to benefit more with added calcium in their water (claims that it isdue to the deposits in thier scales)--perhaps what cheekylemur said about how adjusting the water hardness with magnesium, epsom salts or calcium is what the rumors really mean?

By the way, this information compiled into a research paper would make a great research submittion.

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