Types of Tanks
Freshwater:
Freshwater aquariums are where most people introduce themselves to the hobby of fish keeping and find themselves diving further and further into it - with good reason. Freshwater aquariums are stunning and often easier to maintain than other types of aquariums. Hobbyists usually find the fish to be hardier, which allows parameters to fluctuate before running into any issues. Not only are they hardier, but many freshwater fish are beautiful, and just fun to keep! That being said, there are several main types of freshwater aquariums that people like to keep.
The passive community aquarium is a tank stocked with non-territorial fish that can either be schooling or non-schooling, and usually consists of fish like Tetras, Gouramis, Cory Cats, or Danios to name a few. The passive community is one of the introductory tanks, as these fish tend to be very hardy and not too picky with food types, but also have vivid colors and patterns and can be entertaining to watch interact in a peaceful and calming manner.
The semi-aggressive community tank is a step up in difficulty from the passive community tank, but if done well, it can be a rewarding experience. With semi-aggressive fish, you're at risk of losing some fish to aggression - either by one fish eating another or stressing it out to the point of exhaustion - if you don't stock it correctly. By that, we don't necessarily only mean the types of fish, but the male to female ratio, their sizes, the size of your tank, how well they're fed, and so forth. If done well, your semi-aggressive community can be just as docile as a passive community if given the proper care and attention to details. Some fish considered to be semi-aggressive are Cichlids, Freshwater Sharks, larger Gouramis, and large Catfish.
The Betta tank is everyone's very first fish. That, or the goldfish you won at the carnival. The Betta isn't a misunderstood fish among hobbyists, but is poorly taught on how to properly be cared for by most pet stores , leading it to be misunderstood by beginners and young aquarists. A cup of water is not ideal for a Betta to be in, but there has fortunately been a shift in mood towards Betta's and Betta-keeping and pet stores are now starting to inform new owners on how to keep these stunning little fish healthy and happy. Give a Betta a proper tank and they'll be swimming around with joy - just make sure not to keep two males as Betta fish are extremely aggressive towards each other.
The Goldfish aquarium is an underestimated one among the hobby. Goldfish can be fun to watch and their several sets of colors and patterns make selecting your stock interesting. Goldfish are relatively easy to maintain, but are usually kept with other goldfish only as they prefer colder waters, anywhere from 65-75 degrees. You can mix your goldfish in with other fish if you push the temperatures to their upper limits, allowing you to add a Pleco or certain Catfish types.
The Cichlid tank, whether you choose to stock it with African Cichlids or South American Cichlids, tend to be a species specific tank with the inclusion of a bottom feeder Pleco or Catfish. African Cichlids have over one thousand species, each with vibrant and unique colors and patterns, and are aggressive but do well with each other. They prefer harder water with a pH of 8.2 that won't suit most fish, so if you plan to add a Pleco to your African Cichlid tank you will have to keep your pH at around 7.8-8.0. These are lovely to keep as they're a hardy fish and you can heavily stock your tank without much worry. South American Cichlids prefer a more neutral pH at 7.0, and include the famous Oscar fish that are so commonly kept and loved by many aquarists.
The Planted tank is one of the most beautiful in the hobby. Though it usually needs to be stocked with passive fish, if stocked at all, to avoid any tearing of the plants roots, if you select your plants properly you can stock them with select species of fish. The planted tank has a variety of live plants that add another level of maintenance and care to your tank, but add unexpected elements of fun, satisfaction, and visual beauty that plastic plants often can't offer.
Saltwater:
Anyone who owns a saltwater aquarium has heard from their friends, family, coworkers, or whoever, some variation of the phrase "Wow! I wish I could keep a saltwater tank but they're so difficult and expensive to maintain." Yes, there is some truth behind that statement but it is often blown out of proportion. Saltwater tanks shouldn't be intimidating to an experienced aquarium keeper, they should be the exciting next step. While the fish can be more sensitive to your tank's parameters and picky with food, there is something about the mini ecosystem of a marine tank that is exhilarating to keep and watch over. Plus, who doesn't want to work up to keeping a shark or a stingray. Saltwater fish come in many varying species, offering different shapes, sizes, colors, and behaviors. The hobbyist can decide if they want a tank of cryptic fish, or fish that like to swim around grazing the algae and hanging out in the water column, if not both. On top of the types of fish you can keep, marine environments have a stunning array of invertebrates, including crabs, shrimp, corals, urchins, and many more that all together create a lively and active ecosystem. As is the case with freshwater tanks, there are some commonly kept styles of marine tanks.
The passive community tank is very similar to a freshwater passive community, keeping a stock of non-aggressive marine life that interact well with each other. This is a good introduction if you want to build a coral reef aquarium and will be best kept if you add some live rock. The types of fish you'll find here will have some preferential places in the tank - some will like to dwell on the bottom sifting through the sand, some will be cryptic and find comfort hiding away in your rocks, and others will prefer to hang out in the water column. Common fish you'll find in a passive community like this will be Cromis, Clownfish, Tangs, or Goby's.
The coral reef aquarium is a magnificent slice of the ocean in your home. One of the most desired aquariums, the coral reef aquarium comes in many varieties and the hobbyist can decide what kind of reef they want. The reef aquarium is one that can be difficult to manage and plan, since there are a lot of variables to consider. If your reef has lots of live rock, you will want to keep a cleaning crew of marine invertebrates to keep the algae in check. If you keep soft corals, you will have a larger variety of fish to stock your tank with; if you add hard corals you will have fewer options and will need more light for your aquarium. The balance of a coral reef is well worth the effort, and since you're creating a living, functioning mini ecosystem, it will almost be self-sustaining. Once you get one going, you'll likely watch it for hours on end admiring your animals interacting and enjoying the fruits of your labors.
The semi-aggressive community tank requires some careful planning on how to stock it, since you could suffer some serious loss if stocked recklessly. Some fish you could include in a semi-aggressive community are Butterflyfish, Tangs, Clowns, Rabbitfish, Angels, and Wrasse, among many more - but you wouldn't do them all together. This list includes some invertivores and corallivores, which will pick off your smaller invertebrates or nibble away at your corals if not carefully selected. There is a lot to consider when planning a semi-aggressive community tank, but they allow for some of the most gorgeous species available.
The large community aquarium stocks the larger fish, including species like Tangs, Lionfish, Angels, Eels, Groupers, and larger Wrasse. Given the nature of some of these fish, it will be difficult to stock any corals in a large community tank, so it will be a fish only tank in most cases. These larger fish will have pickier diets, some needing to supplement for the algae and coral they eat in the wild, like Angel fish, or Tangs needing sheets of algae to feed off of. These fish need large aquariums but can display how big and beautiful these marine fish can truly be.