Understanding the effect of the system in marine environments is the key to mastering fish keeping.
One of the leased talked about issues in marine aquarium keeping is the need to facilitate the functioning of a fish's osmoregulation system. The osmoregulation system is responsible for maintaining a healthy balance of fluids and salts within the fish itself.
The higher the amount of salt in the water the harder the osmoregulation system has to work to maintain correct levels of salt and other minerals within the fish, so the higher the osmotic pressure.
The salt levels a fish can tolerate will depend on the environment from which the fish has evolved. Freshwater fish can only handle very low osmotic pressure compared to saltwater fish which are used to withstanding a higher level. Some fish are able to tolerate huge changes in salt level and osmotic pressure allowing them to travel from the salt water oceans, up the waters of freshwater rivers. Fish that commonly live in both fresh and salt water are known as brackish water fish.
There are also many salt water fish that will venture into fresh water at specific points in their lives. Many marine fish such as Salmon will travel right up fast following freshwater rivers to breed allowing their young to grow up in safer stream nurseries away from the threats lurking in the oceans. Once the offspring are large enough they will return to the sea where there is adequate food for them to grow and thrive.
A fresh water fish generally maintains its internal salt level higher than that of the surrounding water. Its gills take in salt from the environment along with oxygen. A large amount of water is constantly diffused into a fresh water fish and excreted as urine so that its system is constantly being flushed. On the other hand, a marine fish has an internal salt level lower than the surrounding water so the osmotic pressure naturally pulls water out of the fish and pushes salt in. Consequently, to maintain the correct salt level, a salt water fish needs to actively excrete salt out of its gills.
A marine fish in water with a slightly higher salt level will devote more molecular energy to osmoregulation; therefore if the fish is suffering from stress lowering the salt level will help the fish to recover. Lowering the salt level will reduce osmotic pressure within the fish allowing it to allocate more of its molecular energy to its immune system, This will help stop the stressed fish from becoming sick and can be used to treat fish that are sick.
The ocean is the most stable ecosystem in the world occupying 77% of the earth's surface area and experiences very little seasonal fluctuations in salt levels in comparison to tiny freshwater lakes and rivers. Most marine fish can only handle about 2 degrees of salt change per day e.g. 1.023 to 1.025. Understating the osmoregulation system can help protect your display aquarium. Every fish you introduce will be initially stressed and it will usually take about 3 weeks for the fish to settle in to a new environment and for its stress levels to return to healthy levels. During this time it's advisable to lower the salt level of your water to about 1.018, to decrease the osmotic pressure experienced by the fish and then gradually increasing it to your previous level (remembering of course to never lower or raise salt levels by more than 0.002 per day). This means that you need to start preparing your tank a few days before you introduce a new fish so that the salt level is right when you bring the fish home Once the fish appear to be doing well for a week you can slowly return the salt level to 1.025 and any time the fish appear stressed again it is worth reducing it to 1.018 again. Many of the corals will not enjoy the lower salt level and may not open up as well during this, but this will not lead to any long term problems.
Another reason to lower your salt level before introducing new fish is that although the ocean is commonly at a salt level of 1.025, and a perfectly run reef aquarium should match this, most aquarium retailers and wholesalers run their aquariums at a salt level of 1.018. This is to allow lower osmotic pressure which in turn reduces the prevalence of diseases among their fish. Because of this most reef tank are actually kept at 1.023 and fish only tanks at 1.020.
A different way to allow fish to gradually adapt to higher salt levels is to use a quarantine tank which can house the fish for 2-4 weeks before it is introduced in the main display tank. This means that the main tank can remain at the desired salt level while also allowing the fish to settle in and de-stress before it is introduced into the display tank. Another advantage of using a quarantine tank is that it reduces the levels of pathogen in your display tank. When fish are stressed, for example when they are being brought home from an aquarium retailer, they release peaks levels of pathogens into the water. These pathogens are not only dangerous to the new fish, but can also make your existing fish sick. If you have a quarantine tank, you can medicate the fish separately and reduce the threat of disease in your new and old fish.
Once the new fish is ready to be introduced into the main tank, you should slowly adjust the salt and pH levels and also the temperature to that of the display tank. This way, you don't have to acclimatize the fish in your display tank but rather easily transfer it across in a net. This quick and simple move from the quarantine tank to the display tank will be stress free compared to what the fish has had to go through to get to you.
Given all the advantages, it is very advisable to use a quarantine tank. However, if the quarantine tank is too small or you cannot provide the correct conditions, then the fish are better off being introduced straight into the main tank.
All the techniques mentioned above can be used whenever any of your fish are sick or stressed, not only at the time they are introduced. You can at any time place a stressed or a sick fish in a quarantine tank with a lower salt level compared with that of your display. Another approach is to perform a freshwater dip on any sick fish or coral. To do this, place the sick marine animal in freshwater with the same temperature and pH as your tank for 10 minutes. This allows the osmotic pressure to kill any pathogens and bacteria that maybe affecting your animal but usually not killing it. In most cases, if the animal does not survive the freshwater dip it was probably not strong enough to survive anyway. The dip can be repeated daily if necessary.
So next time your fish are experiencing stress or illness, help boost their immune system by relieving osmotic pressure.
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