Timothy A. Hovanec, Ph.D.
Many freshwater aquarists avoid trying the saltwater side of the fishkeeping hobby because they have always heard it is “harder.” This means, of course, that having success with a saltwater fish aquarium is harder than a freshwater fish aquarium. I don’t agree that saltwater fish aquaria are harder, but I will say they are less forgiving. I’ll explain why.
There are several differences between saltwater and freshwater aquaria and a bit later I’ll explain some of them and their importance. However, for starters let me say that a saltwater aquarium is no more work than a freshwater aquarium. Since the amount of work is the same, then one can’t be harder than the other, right? So why do people say saltwater is harder? The reason is that if you put off the work in a saltwater aquarium, your fish will suffer, and they are more likely to die sooner than when you put off the work in a freshwater aquarium.
What is the work that has to be done: the filter maintenance and water changes. This really isn’t work, as it is basic aquarium husbandry. For instance, I don’t hear many people saying dogs are work because you have to clean up their droppings or that cats are work because you have to clean out the litter box. So why is cleaning an aquarium filter or changing aquarium water work?
How does not maintaining the filter and not changing the water result in marine fish dying quicker than freshwater fish? The answer lies in a basic difference between the ocean and most freshwater habitats – and I’m not talking salt! The difference is that the ocean surrounding the coral reefs, where most of the fish we get for the hobby live, does not change much from day to day, or year to year. The basic chemical and physical properties of seawater are relatively stable over time, especially compared to many freshwater environments. The salinity, salt concentration and even the make-up of the different chemicals (sodium, chloride, calcium, etc.) in the ocean do not change much over the course of a year. The water temperature may change a little and the pH may go up slightly over a reef during the day, but the fish can swim to other areas in the water that are more suitable. Besides, the differences are not large. Simply put, the ocean is a stable environment in terms of water chemistry.
Contrast that with most freshwater environments. The primary areas which are the original sources of most of the fish we have in the hobby experience a wide range of aquatic chemistry conditions over the course of a year or in even less time. For example, the Amazon has a yearly pattern of flooding and drying that results in a wide range of water chemistry. Freshwater fish have evolved to be able to adapt to a wide set of water conditions.
While freshwater fish do have preferences as to their preferred pH, water hardness or total dissolved solids, the fact is that most can be kept in almost any type of freshwater as long as it does not contain acute poisons such as chlorine. African Lake cichlids prefer hard, alkaline, high pH water but do just fine in conditions much different. Discus from South America prefer soft, acidic, low pH water but grow well in water such as we have in southern California which is hard, alkaline and with a relatively high pH.
Saltwater fish on the other hand have a very narrow range of tolerance to water conditions because they have evolved in the ocean that has changed little. The fish need the pH to be 7.8 to 8.4, the salinity for most needs to be 24 to 34 ppt, and there needs to be the right mixture of seasalts. The range of water chemistry conditions in which saltwater fish can survive is very narrow compared to freshwater fishes.
When you set up an aquarium, the water conditions are good and the fish are fine. However, over time with the establishment of nitrification and the increased amount of fish feed put into the aquarium, the water conditions change, usually for the worse. For example, nitrification, which is a vital process in an aquarium, produces acids that will eventually cause the pH to drop. This happens in both freshwater and saltwater aquaria. Now let’s say that you don’t do any water changes or add buffering agents. What will happen?
In the freshwater system, the pH will drop but most of the fish can tolerate the change and will do OK. Most freshwater fish can live in water with a pH from 5.8 to 8.4. However, if the pH gets too low when you add new fish from the fish store (where they were in aquarium water with a higher, more normal, pH), they may not be able to adapt to the larger difference in pH values and die.
In the case of the saltwater aquarium, by not doing water changes, the pH will drop from the initial value of 8.2 to 8.4. But the fish will not be able to tolerate the change. When the pH gets around 7.8, the fish will start to stress and be susceptible to secondary bacterial infections and die. Saltwater fish cannot live in water with a pH much below 7.6 or so. So if you don’t do the water changes, the saltwater fish will die. Therefore, the thinking goes, saltwater fish are harder. The fact is saltwater aquaria are not harder. They are, however, not for the lazy aquarist.
Many freshwater aquarists get lazy because for many, there is not much of a downside. The fish live for a long time as the water conditions deteriorate in the aquarium. Furthermore, many people think that if they can keep their fish alive for 8 months to a year, that’s great. Of course, the fish should really live 3 to 5 years or more and would have if the aquarist had done some basic maintenance. When these same aquarists try saltwater and continue their lazy habits, the fish are not so tolerant of the laziness and die–leading to the false conclusion that saltwater is harder than freshwater.
Another problem that many freshwater hobbyists have when setting-up their first saltwater aquarium is putting too many fish in the tank. The ocean water around a reef has high dissolved oxygen content and the fish need to be in water with a relatively high amount of oxygen. However, the physical properties of seawater are such that at the same water temperature seawater cannot hold as much oxygen as freshwater. In fact, when saturated, seawater contains roughly only 80% of the oxygen of freshwater. What this means is that you cannot have as many fish in a seawater aquarium as a freshwater aquarium–there isn’t enough oxygen for them.
This can be a disappointment for new saltwater hobbyists who are use to stocking their freshwater aquaria full of fish–you won’t be successful doing that in a saltwater tank. Saltwater tanks require you to accept that fewer fish can be kept in the system.
Another reason saltwater aquaria are “harder” is that they tend to grow more algae than freshwater aquaria and so need to be cleaned more frequently. Why is this? The reason is that in both systems nutrients are produced. These nutrients include ammonia, nitrate and phosphorus based compounds which algae need for growth. And in both systems, aquarists strive to eliminate algae, a type of primary producer, because most hobbyists find them unappealing. However, in freshwater aquaria, another primary producer is available to use the nutrients–plants! In fact, plants compete with algae for the nutrients in the water so an easy way to make sure you never have an algae bloom is to promote the growth of plants. Many aquarists have found this out through trial and error–they know that if their plants are healthy and growing, there won’t be any algae problems.
Unfortunately, in a saltwater aquarium, the choice of plants to compete with the algae is quite limited and thus you find few heavily planted saltwater aquaria. This leaves the nutrients available to the algae that grows unimpeded. Further, in saltwater systems there is a type of bacteria (which use to be called an algae) that goes by the name of cyanobacteria. Cyanobacteria are red in color and can quickly carpet the bottom of an aquarium.
What algae and cyanobacteria have in common, besides being a major headache for saltwater hobbyists, is that they grow in nutrient rich water that contains a lot of organics. How does the aquarist keep a saltwater aquarium from getting too rich in nutrients? By doing water changes. How do you keep the organic levels down? By doing water changes, siphon cleaning the coral substrate, changing the activated carbon and keeping the protein skimmer functioning well.
Once again, if the regular maintenance is done, there will be fewer problems. But by not doing the maintenance, it seems like saltwater aquaria are harder than freshwater aquaria. This is not the case. A little maintenance every 3 or 4 weeks on a saltwater aquarium, which is not overstocked or overfed, will mean success and you’ll find that a saltwater tank is no harder than a freshwater aquarium. Enjoy!
©1999, Timothy A. Hovanec, Ph.D.
Originally published in Aquarium Fish Magazine, Mar. 1999