There are several valid reasons why you might decide to upgrade your freshwater tank but perhaps the most important reason is for the health and safety of your fish. Many novice aquarium hobbyists make the mistake of purchasing juvenile specimens of large species like Oscars and other cichlids, not realizing just how large these fish can get. If you make the mistake of placing an Oscar in a 30-gallon tank, it won’t be long before you either need to re-home your fish or upgrade the tank. Not only is the size of your fish a factor but the number of fish in your tank could also contribute to the need for an upgrade. But with the proper understanding and planning setting up your first saltwater aquarium can be much easier than you had hoped.
Difficulty :The next thing most aquarists looking to get into the marine aspect of the hobby consider is the difficulty. Put simply, successfully keeping a saltwater aquarium has a higher learning curve than an equivalent freshwater tank- it's why marine aquariums aren't recommended as a first foray into keeping fish. Everything there is to know to successfully keep a freshwater tank applies to a saltwater tank- plus more. The biggest hurdle is probably maintaining the water quality. The issue is it's more or less impossible to tell if something is wrong with the water at a glance. Detecting problems requires testing and retesting to see how things are changing over time. None of this is particularly difficult actually, but it does require some diligence for the owner to keep an eye on things. It's something many aquarists, old and new, slack on. Besides the water, another issue is the effort required in meeting the specific needs of the tank's inhabitants. Many marine creatures are less easygoing than their freshwater counterparts. They often have more specific dietary needs and figuring out how various species will interact is tricky. Of course the same could be said of the freshwater side of the hobby as well. On both sides there are plenty of species that are suited for the beginner who's still learning the ins and outs of managing their tank.
Cost :Probably the first consideration people contemplating a saltwater aquarium setup make is the cost. The perceived notion is that a marine tank will cost more than a freshwater tank of comparable size. Well, this notion is largely true. Even a no frills fish only saltwater aquarium requires more equipment to set up and has a higher operating cost than a basic freshwater tank and if you wish to keep corals the cost only goes higher. Of course that's not to say a freshwater tank can't be expensive either. A large planted setup could easily match a reef tank's costs. Really the cost of any setup is a result of the size and complexity more so than what it's meant to keep. Furthermore, no matter what type of tank you're running much of the equipment is the same. A freshwater setup can be converted to saltwater for a relatively low cost since you'll be reusing a lot of what you already have.
Availability :One thing that might not immediately come to mind is the availability of both equipment as well as livestock from local sources. Pretty much anything you may want can be ordered online but this is always less than ideal. Not being able to check out things in person can be a problem and returns or exchanges are of course more of an issue. Furthermore, shipping live animals isn't exactly easy which translates to a higher cost and greater risk of receiving unhealthy specimens. It's definitely not a bad idea to see what's available locally prior to making the decision to set up a saltwater tank if cost is a factor. Do remember that many stores will be willing to order things you may want that aren't currently in stock, however.
Water Changes :Maintenance for either type of setup is basically the same- cleaning the glass, vacuuming up debris, changing water, and so on. However, that last one, changing water, is a bit more complicated when it comes to marine tanks. The reason is of course the salt that needs to be mixed into the change water prior to the actual switch. Mixing up a fresh batch of saltwater isn't hard but it does take time a bit of effort to get the parameters just right meaning water changes have to be planned ahead, usually by about a day. By extension this also means you need to have enough space somewhere near the tank to mix this water and let it sit while the salt dissolves. Finally, water changes for a saltwater tank can be a bit messier, especially in the long run. Any water spilled from a freshwater tank will dry in time more or less. However, water spilled from a marine tank is full of salt and when it dries will leave that salt behind so wiping up any drips is a bit more of a concern.
Many people would like to own and maintain a saltwater aquarium but they shy away from them, turning instead to the freshwater variety because they have been told that saltwater aquariums are difficult to maintain and require additional equipment. That is not necessarily true. For the most part converting a freshwater tank to a saltwater tank is simple. Most of the equipment both tanks use is the same, with only a few notable exceptions.