So You Want a Fish Tank: Fresh or Salt Water?

It’s important to choose whether you are going to go with freshwater or saltwater before you cycle your filtration, so it’s a decision that needs to be made early. It’s also important to choose because you can’t mix freshwater and saltwater fish in one tank.

Why Can’t We Just Mix Fish?

Everyone, and I mean everyone, seems to want “a Nemo”. I’ve been asked about a tank for “a Nemo” (a clownfish) half a dozen times in the last year. Problem is, clownfish are saltwater fish. You can’t just throw them in with your goldfish (for multiple reasons) and expect anything good to come of it.

The reason you can’t mix saltwater and freshwater fish has to do with osmosis. Osmosis is related to diffusion, where molecules move from high concentration to low concentration. If you drop food coloring in a glass of water the food coloring spreads out. This is diffusion. If you blocked the food coloring from moving with some sort of membrane that allowed water to move through it the water would move instead. That would be osmosis. Instead of the food coloring spreading out into the water the water would come to the food coloring.

Now imagine a fish. At a cellular level all a fish is is a bunch of bags of slightly salty water. It’s not all salts, but there’s stuff dissolved in the fish. There’s less stuff dissolved in freshwater and more stuff dissolved in saltwater. So when a fish is placed in freshwater there’s more stuff dissolved in the fish than the surrounding water and water tries to move into the fish. A fish in saltwater faces the opposite problem. Somewhat bizarrely it actually loses water to the surrounding ocean.

Most of this water loss or gain takes place in a fish’s gills. Gills are, at their most basic, thin blood vessels that are surrounded by water. Under the gill cover of a bony fish (the operculum) the gills look like red feathers, each filament of the “feather” being a blood vessel. This works wonderfully for fish because fish blood is normally lower in oxygen than the water (since the fish used oxygen in its cells), higher in carbon dioxide (since the cells produced CO2 in the same process that used oxygen), and higher in ammonia (which we discussed last article). This means that, without the fish doing anything but pumping blood through the gills, oxygen enters the fish’s blood and CO2 and ammonia leave. However, there’s no way to allow these things to diffuse across the blood vessel and not let water through, and so the fish also deals with the loss and gain of some things it would rather not have moving in and out of its body.

However, fish are well-adapted to live in the water. Freshwater fish are adapted to deal with a constant influx of water by having ways to constantly push it back out. Saltwater fish do the opposite. A few fish are euryhaline, meaning they can deal with a wide range of salinities, and these fish can generally change their physiological strategy based on the water they find themselves in. (I still wouldn’t recommend moving a euryhaline fish directly from freshwater to saltwater. It might require hours, days, or even months to switch over.) (A few fish deal with this issue in an entirely different way, where they match the salinity of the water around them, but these aren’t fish you tend to keep in aquaria.)

Now imagine taking a freshwater fish, like an oscar, and dumping it into saltwater. The oscar is pumping water out of its body because it normally has water flowing in. Now water is flowing out, and the oscar is just making it worse. In short order the oscar will be dead. If we move our clownfish into the oscar tank the opposite thing will happen with identical results. The clownfish will pull water in even though water is now rushing into its cells. Its cells will begin to lyse (explode) and it will die.

This grid shows a freshwater fish (an oscar, Astronotus ocellatus) and a saltwater fish (an ocellaris clownfish, Amphiprion ocellaris) in both fresh and salt water. The depth of the blue indicates the salinity of the fish and the surrounding water. The black arrows indicate the direction that water travels by osmosis (towards the saltier area) and the purple arrows indicate the direction the fish moves water to compensate. As you can see, when a fish is in the correct environment the arrows are in opposite directions, which keeps the water level inside the fish constant. When the fish is in the wrong environment the arrows are in the same direction, and so the fish’s own physiology exacerbates the osmotic problem, leading to the fish’s death.
This work by Eric Butler is licensed under a Creative Commons Attribution 4.0 International License.

So don’t mix freshwater and saltwater fish.

Ok, So How Do I Decide?

If you have your heart set on a particular species then you need to go with that water type. But if you’re flexible on exact fish here are a few basic rules.

Saltwater gives you a greater variety of non-fish things to put into a tank. Saltwater gives you the opportunity to go for a reef tank, which is full of corals, crabs, shrimp, tube worms, clams, and perhaps echinoderms like starfish or sea urchins (although be aware that starfish often die rapidly in tanks – at least rapidly for starfish). For some people this is a big draw, and reef fish are very pretty fish. I run a small saltwater tank that is nothing but some easy corals, snails, and crabs. I like it a lot, and the small animals make up for some of the big disadvantages of saltwater tanks. The flip side of this is that some of the odder animals sold for saltwater tanks are so poorly understood that it’s almost impossible to even get accurate advice for keeping them alive.

Saltwater is much more expensive. Not every item and every fish is going to be more expensive, but I have paid between $1-$14 per fish for freshwater fish and between $6-$25 for saltwater fish. I’ve never seen a freshwater fish that I wanted that I thought was too expensive (which is to say I’ve seen expensive fish, they just aren’t frequently seen or interesting to me). I’ve skipped on buying dozens of saltwater fish because of price. Now, maybe I’m just cheap, but this is something to consider. This price difference is due to some of what follows.

Saltwater fish are much harder to keep. One really obvious difference between the ocean and freshwater is size. Oceans make up about 66% of the planet’s surface and freshwater makes up about 0.02%. Saltwater fish live in enormous bodies of water while freshwater fish normally don’t. A lot of the freshwater fish popular in tropical fishkeeping come from small creeks or pools (although, contrary to often-repeated opinion, wild betta fish do not actually live in puddles [although some killifish have been reported breeding in puddles]).

Imagine a small creek running through a tropical area in which there is a distinct wet season and a dry season. In the dry season the water becomes hot, more polluted with fish waste (because there’s less water per fish), and because the water isn’t being replaced it ends up with more of whatever it picks up from the creek bed (which may make it more acidic if the bottom is mostly dead plants, or harder if the bottom is certain sorts of stone). When the rains come the water volume may triple in a day. The new water is colder, cleaner, neutral to slightly acidic in pH, and has nothing dissolved in it. If the fish can’t survive these huge changes in the water they live in they’ll die out as a species. Now imagine the ocean. It doesn’t rain for six months. Nothing changes. It rains for a month straight. Fish right at the surface notice the influx of cold, fresh water, but fish ten or twenty feet down probably don’t even know anything is happening. And so, as a direct result of their environments, freshwater fish tend to be much more tolerant of temperature change, pH changes, water hardness change, and those nitrogenous wastes I talked about in the filtration article. Saltwater fish, on the other hand, are much more likely to die when anything changes. This not only makes the fish more expensive, it also means you need a lot more equipment to keep saltwater fish alive.

You should think about a bigger tank for saltwater. Actually, bigger tanks are easier in many ways (which I’ll discuss in another article), but since saltwater fish come from such large environments the advantages of a larger tank are all magnified for them.

You are also much less likely to be able to breed saltwater fish at home. Maybe you don’t care about this, but if you ever want to try to breed fish freshwater fish are much easier. With guppies the challenge is to prevent them from breeding. A few saltwater fish (like some cardinalfish) have been successfully bred in the aquarium, but most can’t be. (This also means that if you’d like to avoid buying fish that have been caught in the wild, perhaps in ways that aren’t very good for the environment, you have more choices in freshwater.) The reason for this is that the ocean has a very different current system than freshwater. Freshwater basically all flows to the ocean. The ocean, however, has loop currents. A number of oceanic species lay eggs that float on these giant currents, hatch out into larvae that look very different from their parents (and live a drifting life far above the bottom), and eventually circle back home (or another suitable area) where they become “normal looking” and start acting more like the adults. Freshwater fish tend not to do this, since having your eggs blown constantly downstream isn’t such a good idea. This means that plenty of freshwater fish will lay their eggs on the ground or on plants and a fish keeper can take care of them, whereas many saltwater fish lay eggs that just go straight into the filter with all the other floating debris.

My quick advice would be not to go for saltwater unless you really, really want a particular fish (or are rich and retired). Saltwater tanks are more expensive and much more work.

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