Aquarium Keeping

This deck introduces the foundational concepts behind keeping an aquarium, starting with what an aquarium actually is and the broader hobby of fishkeeping. You'll work through the main categories of tanks, including freshwater, saltwater, and brackish setups, and explore popular styles like community tanks, planted tanks, biotopes, species-only tanks, and reef tanks. The cards also cover the essentials of water chemistry, with a focus on the nitrogen cycle and how ammonia is produced and converted in a healthy tank.

It's a great starting point if you're new to the hobby and want to understand the terminology before setting up your first tank, or if you've recently started keeping fish and want to build a stronger knowledge base. The questions assume no prior experience, so absolute beginners can work through them from start to finish without feeling lost.

Because many of these concepts build on one another — especially the steps of the nitrogen cycle — it helps to study the related cards together rather than in isolation. Spacing your review sessions over several days will give your memory time to lock in the new vocabulary, and trying to explain a term in your own words before flipping the card is a good way to check that you really understand it.

An aquarium is an enclosed tank that holds water, aquatic life, and often plants. The hobby of maintaining these systems is called fishkeeping, and it spans everything from small desktop bowls to large living-room reef installations. The two broadest divisions are freshwater and saltwater tanks. Freshwater setups contain no salinity and are generally simpler, while saltwater tanks require sea salt mixed to a precise specific gravity and demand more complex equipment. Between these extremes sits brackish water, which has a low salinity somewhere between fresh and salt and is used for species from estuaries and mangrove habitats.

Beyond water type, aquariums fall into several major categories. A community tank combines peaceful, compatible species. A planted tank prioritizes live aquatic plants as both decoration and biological support. A biotope replicates a specific natural habitat, such as an Amazonian stream or a Lake Tanganyika shoreline, often using fish and decor from that same region. A species-only tank houses a single species, usually for breeding or specialized care. In the saltwater world, a reef tank is built around corals and other reef-associated life, distinct from a fish-only saltwater setup.

For newcomers, one principle shapes every choice that follows: bigger is more stable. A larger volume of water resists swings in temperature, pH, and waste concentration, so a 20-gallon or larger tank is almost always easier to manage than a smaller one. Beginning with a sturdy starter setup of tank, filter, heater, light, substrate, decor, and dechlorinator allows the hobbyist to focus on learning rather than rescuing a fragile system.

The single most important concept in fishkeeping is the nitrogen cycle, the biological process that converts fish waste into less harmful compounds. Fish excrete ammonia through their gills and waste, and uneaten food and decaying matter add more. Ammonia is highly toxic to fish even in tiny amounts, so it must be removed. Beneficial bacteria of the genus Nitrosomonas colonize the filter and substrate, converting ammonia into nitrite, which is also toxic because it interferes with oxygen transport in fish blood. A second group of bacteria, Nitrobacter, then converts nitrite into nitrate, which is far less harmful and can be kept at safe levels through water changes and plant uptake.

Before fish are added, a new tank must be cycled, meaning the bacterial colonies must be established. This typically takes two to six weeks. The safer modern approach is fishless cycling, in which pure ammonia or small amounts of fish food are added to feed the bacteria without exposing animals to toxins. Fish-in cycling, by contrast, uses hardy fish to produce ammonia, but it is considered outdated because it stresses and can kill the animals involved. A faster alternative is the quick-cycle method, which introduces mature filter media from an established tank or commercial bacteria products to seed the new system.

The target readings once a cycle is complete are ammonia at zero, nitrite at zero, and nitrate below roughly 40 ppm. Tracking these numbers with a reliable test kit during cycling tells the hobbyist exactly when it is safe to begin stocking. Many of the worst disasters in the hobby trace back to a tank that was never properly cycled, making patience during this stage one of the most valuable habits a new keeper can build.

Beyond the nitrogen cycle, several other water parameters shape fish health. pH measures acidity on a scale where 7 is neutral, lower values are acidic, and higher values are basic. Many tropical community fish thrive between 6.8 and 7.5, while African cichlids from the rift lakes prefer harder, more alkaline water in the 7.8 to 8.5 range. South American fish such as tetras and discus often do best in softer, slightly acidic water between 6.0 and 7.0. General hardness, or GH, measures dissolved minerals like calcium and magnesium, while carbonate hardness, KH, acts as a buffer that resists sudden pH swings. A useful rule is that stable pH is more important than a perfect number, because most fish tolerate moderate values better than constant fluctuation.

Temperature is equally critical. Most tropical fish do best between 72 and 82°F (22 to 28°C), maintained by a reliable heater sized at roughly 5 watts per gallon, with adjustment for cooler rooms. Coldwater species such as goldfish and white cloud mountain minnows prefer temperatures below 72°F and usually do not need a heater at all. A simple thermometer lets the keeper verify heater accuracy daily, since even small deviations can stress inhabitants over time. The principle that warmer water holds less oxygen also matters: heavily stocked warm tanks benefit from surface agitation to keep dissolved oxygen high.

Routine maintenance keeps these parameters in balance. Regular water testing catches problems early, and the adage to test rather than guess reflects how unreliable visual estimation can be. Most tanks do well with a 20 to 30 percent water change each week, and new water should always be matched in temperature to avoid shocking the fish. A dechlorinator neutralizes chlorine and chloramine, the latter requiring a product specifically designed to break it down. Long-running tanks can drift into what hobbyists call old tank syndrome, where nitrates accumulate and minerals are depleted, so even mature systems benefit from ongoing testing rather than being assumed stable.

Filtration is the mechanical and biological heart of the aquarium. It removes debris, supports the bacteria of the nitrogen cycle, and may include chemical media to polish the water. Common filter types include hang-on-back (HOB) units, which sit on the tank rim and are easy to maintain; canister filters, which sit outside the tank and offer high capacity for larger setups; simple sponge filters driven by air pumps, which provide gentle flow ideal for breeding tanks; internal filters placed inside the aquarium; and sumps, secondary tanks beneath the display that hold equipment for larger systems. Flow rate is usually sized at four to ten times the tank volume per hour to keep water moving and oxygenated.

Filter media falls into three functional categories. Mechanical media, typically sponges or floss, physically trap particles so they can be rinsed out. Biological media, such as ceramic bio-rings or porous sponge, house the bacteria that convert ammonia and nitrite. Chemical media, most commonly activated carbon, adsorbs certain dissolved compounds, odors, and leftover medications after treatment, though it is not always necessary in every tank. Within the filter, biological media should be gently rinsed in old tank water rather than tap water, since chlorine kills the bacteria that make the cycle work.

Heating and substrate round out the core equipment. A heater matched to the tank size maintains a stable warm temperature, verified daily with a thermometer. Substrate can be gravel, sand, or specialized planted substrates, usually laid one to three inches deep; it provides aesthetic appeal, a surface for beneficial bacteria, and a rooting medium for plants. Decor such as rockwork creates structure and territories for fish, while driftwood adds natural aesthetics and can gently lower pH through released tannins. Because fresh driftwood releases these brown organic compounds into the water, it should be soaked or boiled beforehand, and hobbyists who enjoy the tea-stained effect sometimes build blackwater aquariums that mimic Amazonian habitats.

Live aquatic plants do far more than decorate. They consume nitrates, produce oxygen, compete with algae for nutrients, and offer hiding places for shy or juvenile fish. Beginner-friendly species include Java fern, which should be attached to wood or rocks rather than buried because its rhizome will rot underground; Anubias, a slow-growing, low-light plant; Cryptocoryne, an adaptable genus that often sheds old leaves in a process called Crypt melt before regrowing; Vallisneria, a tall grass-like background plant; and Amazon sword, a centerpiece broad-leaf species. These plants tolerate a wide range of conditions and form the backbone of most low-tech setups.

Aquascaping, the art of designing planted aquariums, has produced several recognizable styles. The Iwagumi style is a Japanese minimalist approach built around carefully arranged stones and short carpeting grasses. The Dutch style uses densely planted, structured groups of contrasting plants in terraced rows. Nature style, popularized by Takashi Amano, takes a naturalistic, asymmetric approach that emulates wild landscapes. Demanding setups, often called high-tech, add pressurized carbon dioxide, strong lighting, and full fertilizer dosing to fuel rapid growth. Low-tech planted tanks rely on easy plants, lower light, no CO₂, and fewer water changes. The Estimative Index approach illustrates an important principle: with adequate light, fertilizer, and CO₂, plants consume nutrients so aggressively that heavy water changes are required to reset the system.

Algae is a natural part of any aquarium but can overtake a tank when light, nutrients, CO₂, and plants fall out of balance. Common types include green dust, green spot, hair, the stubborn black beard algae, and brown diatoms that frequently appear in new tanks and clear as the system matures. Control comes from rebalancing these factors rather than relying on a single fix. Helpful algae eaters include Otocinclus catfish, which need mature tanks with established biofilm; nerite snails, which consume algae but do not reproduce in freshwater; Amano shrimp, named for Takashi Amano and considered excellent grazers; and Siamese algae eaters for larger community setups.

Choosing fish wisely begins with research and a healthy skepticism toward the old rule of one inch of fish per gallon. That guideline ignores differences in bioload, the amount of biological waste each species produces, as well as behavior and territory needs. Every potential species should be evaluated for adult size, temperament, schooling requirements, dietary preferences, and water parameters. Schooling fish such as neon tetras, cardinal tetras, rummynose tetras, and harlequin rasboras must be kept in groups of at least six to feel secure and display natural behavior. Livebearers like guppies, platies, mollies, and swordtails give birth to free-swimming young, and keeping two to three females per male reduces harassment of any single fish.

Popular community fish include tetras, rasboras, danios, some gouramis, corydoras catfish, and livebearers. Larger or more striking centerpiece fish might be angelfish, dwarf gouramis, pearl gouramis, or German blue rams. Bottom dwellers such as corydoras, kuhli loaches, and otocinclus round out the tank by occupying the substrate. Plecos are a special case: the common pleco grows to 18 to 24 inches and needs an enormous tank, while the bristlenose pleco stays around five inches and is a much better choice for most home aquariums. Bettas are usually kept one male per tank because they will fight, though female bettas can sometimes be housed in groups of five or more in larger, well-structured sororities.

Feeding should be done two to three times daily in small amounts that fish consume within a couple of minutes, and the rule of underfeeding rather than overfeeding protects water quality. A varied diet of flakes, pellets, frozen, freeze-dried, and occasional live or vegetable foods mimics natural nutrition. New arrivals must be acclimated slowly, ideally by dripping tank water into the transport bag over 30 to 60 minutes, so they adjust to both temperature and chemistry. Quarantining new fish for about 30 days in a separate tank is one of the most effective ways to prevent disease outbreaks in the main display.

Some fishkeeping paths branch off into more specialized territory. Reef tanks represent the demanding end of the saltwater hobby, building a living coral ecosystem rather than just a fish display. They require a protein skimmer to remove organic waste before it breaks down, return pumps and powerheads for strong water flow, spectrum-tuned high-output LEDs, a heater, and often controllers that monitor parameters around the clock. Corals themselves are animals rather than plants and are divided into soft corals, large polyp stony (LPS) corals, and small polyp stony (SPS) corals, each with different lighting and flow needs. PAR (photosynthetically Active Radiation) targets typically range from 100 to 300 for soft corals and 250 to 450 for SPS. Reef keepers also dose calcium, magnesium, and alkalinity to support coral skeletal growth, and many use a sump with a refugium section filled with macroalgae such as chaeto to export nutrients.

African cichlids from Lakes Tanganyika, Malawi, and Victoria are another specialty, known for vivid color and active behavior. They thrive in hard, alkaline water with pH between 7.8 and 8.5 and appreciate elaborate rockwork that provides territory and breaks lines of sight. South American cichlids occupy a different niche: angelfish, Severums, and Oscars are popular, while discus stand apart for their round, colorful bodies and demanding requirements, including warm, clean water between 82 and 86°F (28 to 30°C). Saltwater parameters for fish-only and reef tanks include a specific gravity between 1.023 and 1.025 measured with a hydrometer, a pH of 8.1 to 8.4, and temperatures around 75 to 78°F.

Coldwater fishkeeping centers on goldfish and koi. Goldfish need cool water, strong filtration, and more space than most beginners expect, with fancy varieties such as orandas and ranchus requiring 20 to 30 gallons or more and common goldfish needing far larger tanks. With proper care, goldfish can live 10 to 20 years or longer. Koi push this further into outdoor ponds of 1,000 gallons or more, where they can reach impressive sizes and live for decades. Across all these specialties, the same lesson applies: research the specific needs of the fish before acquiring them and build the system around those requirements rather than trying to retrofit an unsuitable tank.

Beyond equipment and biology, fishkeeping is shaped by a particular mindset. Patience is the throughline: aquariums reward slow, steady additions and careful observation rather than impulse. Many fish die because they were purchased on a whim without research into tank size, water requirements, or temperament, and the hobby's collective wisdom is to research before buying. Stability beats complexity, so matching the tank to the time a keeper can actually commit is more important than running an elaborate high-tech system that cannot be maintained. A larger tank is generally easier than a small one because greater water volume resists the parameter swings that destabilize tiny setups.

An aquarium is most rewarding when treated as an ecosystem rather than a sterile box. Live plants, balanced fish loads, stable parameters, and regular water changes create biological resilience. A journal that tracks parameters, additions, and problems helps the keeper notice patterns over time, and photo documentation captures how a tank evolves as plants grow and fish develop. The community around the hobby, from local clubs to online forums, is one of its greatest resources, offering shared knowledge when something goes wrong.

The lighthearted joke that the second tank is inevitable, sometimes called multi-tank syndrome, hints at how the hobby tends to grow on people. Some keepers eventually build dedicated fishrooms housing many tanks together, while others pursue breeding projects, from easy livebearers that self-reproduce to egg-layers that require conditioning, specific triggers, and separation of fry. The joy of the hobby comes from observation: the slow rhythm of an aquarium is meditative, the living art changes daily, and a healthy system is its own quiet reward. Stable water and patient care remain the foundation of every thriving tank.