Freshwater fish
Fresh water fish normally drink very little water but absorb copious amounts of water through their gills and excrete it through their urine. Depending on many factors, the water entering the fish’s gills can approach 50% of the total water in the fish every hour.
Fish need electrolytes just like humans, and plasma NaCl, the concentration of sodium chloride (salt) in the liquid part of blood is crucial for maintaining fluid balance, blood pressure, and nerve function.
Since their blood plasma is hypertonic, salt tends to flow out of the fish, and so if water passes through the fish too fast, it can lead to salt depletion.
Like humans, the solute level of their blood plasma is around 300 mOsm/kg, while fresh water is around 5 mOsm/kg. Water will flow in the direction of the higher solute level due to osmosis, especially through their highly exposed gills, but also through their skin. Missing scales can speed up the inflow of water through the skin.
The fish’s kidneys have the role of eliminating large quantities of water while trying to limit salt loss.
Hormones
The fish have three different hormones that are important to osmoregulation.
Arginine vasotocin (AVT) is released to increase the production rate of urine.
Prolactin is released to lower the salt and water uptake of water by the gills and intestines, but a change in release speed requires a morphological change, making this hormone more relevant in chronic osmotic stress. Measuring prolactin levels in a fish can also identify chronic osmotic issues.
Catecholamines are released at times of stress and increase circulation to the gills to improve oxygen uptake. As a result, water intake rises significantly, requiring increased urine production to deal with it, leading to salt depletion.
Salt
A fish’s blood is similar to humans at 0.9% salt. Adding salt to a tank at 5 g/L will make the water 0.5% which would be isotonic to blood. Adding salt to a tank at 10 g/L would make the water hypertonic to blood.1
Male betta tolerance to salt is high, with a 15 day LC50 over 0.8% and an LC100 of 0.75%.2 The 96 hour LC50 of female betta was 1.188% and the 18 day LC50 of female betta was 0.935%. The LC50 survival time was significantly shorter at 1.5%. The 18 day LC100 for female betta is between 0.6% and 0.7%.
Female betta fish kept at 0.9% salinity showed a significant reduction in feeding until the 11th day of the experiment, at which time the female betta had acclimated to the salinity. It is believed that the fish have a hard time taking in water at the higher salinity, slowing digestion time.3
During transport, betta stored in 0.5% salt solution had a significantly lower stress level.4
Dropsy
“Dropsy” is a term describing aedema, or collection of water in tissue. It often results in raised scales of the fish, making them look like a pinecone, so some people call it pineconing. Another technical term for this condition when discussing osmoregulation is hypervolemia.
When a fish has dropsy, it is often caused by a disease of the gills or kidneys, as these two organs are involved in osmoregulation. It is made worse by stress because of the elevated levels of catecholamines causing a faster influx of water. If the underlying disease is an acute infectious one, it may be cured by anti-biotics for bacterial infections, or probiotics or vitamin C to boost the immune system against viral infections.
If a fish has suffered kidney failure, it is unlikely to recover from the dropsy regardless of treatment.
However, the most common cause of dropsy is by transferring a fish from hard water to soft water too fast. Described as acute mortality after large (50% or greater) water change, fish suffer from Acute Ulceration Response (AUR), Environmental Shock, or Delayed Mortality Syndrome (DMS).5
As described above, if a fish is in hard water with a solute level closer to 300 mOsm/kg than the normal 5 mOsm/kg, the fish will have adapted to the slower influx of water by lowering its production of AVT.
When the fish is suddenly placed into lower solute water, the water flows into their body way faster than they’re prepared for, resulting in dropsy.
Treatment
To help lower the amount of water that the fish has absorbed through osmosis, you can raise the solute level of the water to get it to flow in the other direction. You can do this by putting the fish in 1% salt water solution which will have a 342 mOsm/kg, helping the fish to slowly lower its osmotic pressure. This also has the added benefit of reversing the transfer of salt, as the fish is likely salt depleted. The salt will pass from the 1% solution to their 0.9% blood plasma.
There are other treatments. If your fish is stopped up because of overfeeding and the fish can’t expell urine from its vent, which causes dropsy, you might instead use 5 grams of salt and 5 grams of magnesium sulfate per liter which will have a similar solute level.
You can also add 35 ml (7 cap fulls) of Seachem Replenish to a 0.5% salt solution, which results in a similar solute level.
Literature Cited:
1 Greenwell MG, Sherrill J, Clayton LA. Osmoregulation in fish. Mechanisms and clinical implications. Vet Clin North Am Exot Anim Pract. 2003 Jan;6(1):169-89, vii. doi: 10.1016/s1094-9194(02)00021-x. PMID: 12616839.
2 Deboleto, Sandriele & Santos, Rudã & Souza, Rebeca & Honorato, Claucia. (2020). TOLERÂNCIA CRÔNICA DE BETAS (Betta splendens) MACHOS A ÁGUA ACRESCIDAS DE SAL. Revista Científica Rural. 22. 251-258. 10.30945/rcr-v22i1.2700.
3 Zuanon, Jener & Salaro, Ana & Veras, Galileu & Tavares, Mateus & Chaves, William. (2009). Tolerância aguda e crônica de adultos de beta, Betta splendens, à salinidade da água. Revista Brasileira De Zootecnia-brazilian Journal of Animal Science – REV BRAS ZOOTECN. 38. 10.1590/S1516-35982009001100005.
4 Sintuprom, C., Nuchchanart, W., Dokkaew, S., Aranyakanont, C., Ploypan, R., Shinn, A. P., … & Chatchaiphan, S. (2025). Effects of Confinement Rearing and Sodium Chloride Treatment on Stress Hormones and Gene Expression in Siamese Fighting Fish (Betta splendens). Journal of Applied Animal Welfare Science, 1-17.
5 Noga, E.J. (2010). Fish Disease: Diagnosis and Treatment, Second Edition. Wiley-Blackwell: Ames, IA. pp. 104, 325.