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A fish tank is a glass or acrylic container that holds water and aquatic life, with a filter, heater, and lighting to keep the water livable. You need all four of those — tank, filter, heater, light — plus a substrate and a cycled filter before any fish go in. A bowl is not a fish tank: it has no filtration, no heater, and too little surface area for gas exchange, so it is only suitable for a few very tolerant species, if anything. Saltwater tanks add live rock, a protein skimmer, and reef-specific lighting, and if you want corals, the setup requirements rise again.

What separates a fish tank from a bowl

The container itself is the least important part. What makes a tank a tank is the life-support system attached to it:

• Filtration — mechanical, biological, and often chemical. The biological part is the one that matters most.
• Heating — most tropical fish need a stable 24–26 °C (75–79 °F). A bowl swings with room temperature.
• Lighting — for plants, corals, and your ability to see the fish.
• Surface area and volume — more water dilutes waste and buffers temperature swings. A larger tank is easier to keep stable than a small one, which surprises most beginners.

The essential equipment

Live rock in a saltwater tank is not decoration. It is the primary biological filter, hosting the bacteria that process fish waste.

The nitrogen cycle, and why you cycle first

Fish waste and uneaten food break down into ammonia, which burns gills and kills fish. A filter does not remove ammonia by itself — bacteria living in the filter media do. Those bacteria take weeks to establish.

The cycle runs in three stages:

1. Ammonia rises from waste or an added source.
2. Nitrite appears as bacteria convert ammonia. Also toxic.
3. Nitrate appears as a second group of bacteria converts nitrite. Much less toxic, removed by water changes.

You cycle a tank by running it with a waste source but no fish — fishless cycling with ammonia, or a pinch of fish food — and testing daily until ammonia and nitrite both read zero and nitrate is present. That typically takes two to six weeks. Adding fish before this point is the single most common reason new tanks fail.

Basic maintenance

• Water changes — 10–20% weekly or fortnightly, replacing water with dechlorinated (freshwater) or pre-mixed saltwater at matching temperature and salinity.
• Testing — ammonia, nitrite, nitrate, pH, and for saltwater also salinity and alkalinity. Test more often in the first few months.
• Filter cleaning — rinse media in old tank water, never under tap water, which kills the bacteria.
• Glass and equipment — algae scraping, impeller cleaning, and checking the heater.

Freshwater versus saltwater, and where corals fit

Freshwater is the cheaper, more forgiving starting point. Saltwater costs more, needs a refractometer and salt mix, and punishes mistakes faster — but it is what lets you keep marine fish and invertebrates.

Corals sit at the demanding end of that spectrum. They need stable parameters, reef-grade lighting, and consistent calcium and alkalinity. If you want corals, plan the tank around them from the start rather than upgrading later. Tank-raised, aquacultured corals — grown on real rock in captivity rather than collected from a reef — are generally hardier in home aquariums and easier to place than wild-collected specimens, and they avoid the collection impact on wild reefs. That makes them the sensible first choice for a new reef tank, though they still need the same stable water conditions as any other coral.

Deciding what to set up

If you want a low-maintenance tank with colourful fish, a freshwater setup of 60–100 L with a canister filter and heater is the straightforward path. If you want a reef, budget for a larger tank, a skimmer, reef lighting, and a longer cycling and stabilising period before you add corals. Either way, the sequence is the same: set up equipment, cycle the tank, test until stable, then add livestock slowly.