Fish & Aquarium Care Guide

Written and reviewed by Hugo Mellado · Last updated August 2026

Fishkeeping looks like decorating and is actually chemistry. The tank is a small ecosystem, and your job is to establish and protect the bacteria that convert fish waste into something harmless. Almost every death a beginner experiences comes from starting too fast, stocking too heavily or changing water too rarely.

Start here: cycling the tank

Before any fish arrives, the aquarium needs a colony of beneficial bacteria living in the filter and on surfaces. Ammonia from waste is converted to nitrite, then to far less toxic nitrate, which you remove with water changes. Establishing that cycle in a new tank takes roughly two to six weeks.

• Set up the tank, filter, heater and substrate and let it run.
• Add a small, measured ammonia source, or bottled beneficial bacteria, or filter media from a healthy established tank.
• Test every day or two with a liquid test kit for ammonia, nitrite and nitrate. Paper strips are not accurate enough for this stage.
• The tank is cycled when ammonia and nitrite both read zero and nitrate is present.
• Add fish gradually, a few at a time, so the bacteria can keep pace.

Never wash filter media under tap water — chlorine kills the colony you spent weeks growing. Rinse it in a bucket of tank water instead.

Nutrition

Feed less than you think. Most community fish need one small feeding a day, or two very small ones, sized so everything is eaten within a couple of minutes. Uneaten food decays, spikes ammonia and feeds algae. A fasting day each week does no harm to healthy adult fish.

• Match the food to the mouth: floating flakes and micro pellets for mid-water fish, sinking wafers for bottom dwellers such as corydoras and plecos.
• Rotate quality staples with frozen or freeze-dried brine shrimp, daphnia and bloodworms.
• Feed vegetable matter — blanched zucchini, spinach or algae wafers — to plecos, otocinclus and many barbs.
• Betta fish are carnivores and need a betta-specific pellet, not a plant-based flake.
• Store food dry, sealed and cool, and replace it every six months; vitamins degrade.

Habits & Behavior

Behaviour is your best water-quality gauge. Fish gasping at the surface, hanging listlessly near the filter outflow, clamping their fins or darting and rubbing against decor are usually reporting ammonia, nitrite, low oxygen or a parasite — not personality. Test the water first, before medicating anything.

Stock socially as well as numerically. Schooling species — neon tetras, harlequin rasboras, corydoras, zebra danios — are stressed and nippy in groups of two or three, and calm in groups of six or more. Male bettas cannot be housed together. Angelfish and many cichlids become territorial as they mature, and a peaceful juvenile community can turn hostile in a few months.

Research adult size before buying. The common pleco reaches well over a foot, goldfish are large, long-lived, cool-water fish that quickly outgrow a bowl, and the small shark-shaped fish in the store display becomes a foot-long territorial adult. Overstocking is the second most common beginner mistake after skipping the cycle.

Care & Maintenance

A bigger tank is easier than a small one, because volume buffers mistakes. Twenty gallons is a far friendlier starting point than five. Choose a filter rated for more than your volume, add a heater with a separate thermometer for tropical species, and give the surface some movement for gas exchange.

Weekly routine

• Change 20 to 30 percent of the water with a gravel vacuum, which removes waste and nitrate at the same time.
• Treat replacement water with a dechlorinator, and match the temperature before adding it.
• Test nitrate, and pH if you keep sensitive species.
• Wipe the glass, trim plants and remove dead leaves.
• Check the heater and filter are actually running — a silent failure is easy to miss.

• Monthly: rinse filter sponges in old tank water; replace only part of the media at a time.
• Lighting: six to eight hours a day is plenty for a low-tech planted tank. Longer photoperiods mostly grow algae.
• Temperature: most tropical community fish sit between 74 and 80 °F; goldfish and white cloud minnows prefer cooler water.
• Quarantine: new fish in a separate tank for two to four weeks whenever possible.
• Top-ups: replace evaporated water with dechlorinated fresh water, but remember evaporation is not a water change.

Games & Enrichment

Fish use the space you give them. A bare tank with a plastic plant is stressful for species that evolved in dense vegetation or shaded roots.

• Plant live stems and easy species — anubias, java fern, cryptocoryne, vallisneria — for cover, oxygen and nitrate uptake.
• Add driftwood, rock caves and leaf litter so shy fish and bottom dwellers have territory and hiding places.
• Leave open swimming lanes for active schooling species, and floating plants for surface dwellers such as bettas and hatchetfish.
• Vary feeding: sinking food for foragers, a slice of vegetable clipped to the glass, occasional live food.
• Keep hardscape stable and smooth; sharp edges tear fins, and unstable rocks fall.

Health & Veterinary Tips

Fish medicine begins with water testing. Keep a liquid test kit, a log of results, and a small quarantine or hospital tank. Most illnesses are opportunistic and take hold when water quality or temperature swings.

• New tank syndrome: ammonia or nitrite above zero. Do daily partial water changes and stop feeding heavily until the readings fall.
• Ich: white salt-like spots and flicking. Treated with gradual warming and an appropriate medication, following label directions carefully with scaleless fish and invertebrates.
• Fin rot: ragged, receding fins, usually secondary to poor water quality. Clean water first, medication second.
• Swim bladder trouble in fancy goldfish and bettas: often diet-related. Fast briefly, then feed soaked, sinking food.
• Algae blooms: reduce light hours and feeding, increase water changes, add fast-growing plants.
• Sudden multiple deaths: test immediately, check the heater, and consider whether an aerosol, cleaning product or unrinsed decoration entered the room or the tank.

For persistent or unusual disease, aquatic veterinarians do exist and can be found through veterinary associations — a valuable option for koi ponds and larger investments.

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