Frequently Asked Questions

Start with 20 gallons (76 litres) if you have the space and budget. A tank that size holds temperature and water chemistry steady, so a mistake takes hours to become dangerous instead of minutes. Small tanks swing faster: a 5 gallon (19 litre) tank can go from safe to a toxic ammonia reading overnight, because there is so little water to dilute waste. Bigger volumes are more forgiving, and the weekly maintenance is almost the same either way. If 20 gallons (76 litres) is too large for your room or wallet, 10 gallons (38 litres) is a workable floor for a few small fish. Avoid anything under 5 gallons (19 litres) for a first tank.

Easier, which surprises people who assume a small tank means less work. A larger volume dilutes fish waste, holds a stable temperature, and gives you reaction time when something goes wrong. In a 40 gallon (151 litre) tank an ammonia spike climbs slowly and a heater fault cools the water over many hours. In a 3 gallon (11 litre) desktop tank the same problems hit in a fraction of the time. The trade-offs are cost, floor weight, and the space it takes up. A filled 40 gallon (151 litre) tank weighs over 400 pounds, so it needs solid furniture. For upkeep, a big tank and a small tank both come down to one weekly water change.

Plan for the tank plus a heater, a filter, a lid, a light, substrate, a water conditioner, and a liquid test kit. Buy the test kit and the water conditioner first, because you cannot run a tank safely without them. You do not need a fancy light, extra pumps, or chemical additives to start. Secondhand tanks are common and cheap, and a used glass tank is fine as long as it holds water and the silicone seals are intact. Spend on the filter and heater, and save on decor. Prices shift by country and season, so I will not quote figures that would be wrong for half of readers. Set your budget around the hardware list above.

A dedicated aquarium store is usually the best first stop, because the staff can test your water and the livestock tends to be healthier than in a general pet aisle. Online retailers work well for hardware like heaters, filters, and test kits, often at lower prices than a physical store. For live fish, buy local so you can see the animal before it goes in a bag and skip the shipping stress. Look at the shop's own tanks first: cloudy water, dead fish left on display, or fish with clamped fins are reasons to walk out. Secondhand marketplaces are good for tanks and stands. Whatever the source, quarantine new fish if you already have an established tank.

Plan for four to six weeks between filling the tank and adding fish, because the tank has to cycle first. The physical setup takes an afternoon: rinse the substrate, fill with dechlorinated water, run the heater and filter, and check for leaks. The wait is biological. Your filter needs to grow the bacteria that convert fish waste into safer compounds, and that colony takes weeks to build. Adding fish on day one is the most common beginner mistake, and it causes most early deaths. If you want fish sooner, a fish-in cycle is possible with daily testing and water changes, but a fishless cycle is safer and less work. Warm water near 78°F (26°C) speeds the wait along.

Cycling means growing a colony of beneficial bacteria in your filter that turns toxic fish waste into a safer form. Fish produce ammonia, which is poisonous even at 0.25 ppm. One group of bacteria eats ammonia and produces nitrite, which is also toxic. A second group eats nitrite and produces nitrate, which is relatively safe at low levels and removed by water changes. Until both groups are established, ammonia and nitrite build up and poison the fish. This whole process is the nitrogen cycle. A cycled tank reads 0 ppm ammonia, 0 ppm nitrite, and some nitrate. You cannot see the bacteria, so a test kit is the only way to track progress.

Four to six weeks for a fishless cycle at a stable 78°F (26°C). Warmth speeds the bacteria along, so a cold tank cycles slower. You can cut the wait to one or two weeks by adding filter media, gravel, or a sponge from an established healthy tank, which seeds the bacteria you need. Bottled bacteria products can help, but results vary by brand and freshness, so treat them as a boost rather than a promise. There is no reliable way to cycle a tank in a day, and anyone claiming an instant cycle is selling something. Test every two or three days and watch ammonia rise, then nitrite rise and fall, then nitrate appear.

Yes, but it is riskier and more work, and it is called a fish-in cycle. If you already have fish in an uncycled tank, test ammonia and nitrite daily and change 25 to 50 percent of the water whenever either reads above 0 ppm. Use a dechlorinator and match the new water's temperature to the tank. Feed lightly, because every uneaten flake becomes more ammonia. Pick hardy fish if you go this way, and add only a few. A fishless cycle avoids all of this by letting you grow the bacteria with a bottle of ammonia instead of live animals. If your fish are already home, do not wait, start the daily testing and water changes today.

Your tank is cycled when it reads 0 ppm ammonia and 0 ppm nitrite while showing some nitrate, and holds those numbers after you add an ammonia source. The standard check: dose ammonia to 2 ppm, or feed a small pinch, wait 24 hours, and test. If both ammonia and nitrite read 0 ppm a day later, the bacteria process waste fast enough for fish. Nitrate sitting at 5 to 40 ppm confirms the cycle is running. Do not judge by water clarity, because clear water can still be full of ammonia. A liquid test kit is the only proof. Once cycled, keep feeding the bacteria with fish or ammonia so the colony does not starve.

No, a tank will cycle on its own, but bottled bacteria can speed it up. The bacteria you need drift in from the air, the fish, and the plants, and they multiply once there is ammonia to eat. A bottled product adds a starter dose so the colony reaches full strength sooner, sometimes shaving a week or two off the wait. How well it works depends on the brand, how it was stored, and whether it is past its date, so it is a helper rather than a sure thing. The most reliable shortcut is a used filter sponge or a cup of gravel from a healthy established tank, which carries a live, proven colony straight into your filter.

Four matter for a beginner: ammonia, nitrite, nitrate, and pH. Ammonia and nitrite should always read 0 ppm in a cycled tank, and any reading above 0 ppm needs action that day. Nitrate is safe up to about 40 ppm and tells you when to change water. pH matters mostly for stability, since most community fish adapt to a steady 6.5 to 7.5, and a stable wrong number beats one that swings. You can add general hardness and carbonate hardness tests later to help with plants and pH stability, but they are not day-one buys. Temperature is a fifth reading, taken with a thermometer rather than a chemical. Those five cover almost every beginner situation.

Buy a liquid test kit. Paper strips are quick and cheap, but they are less accurate and often read ammonia and nitrite poorly, which are the two numbers that keep fish alive. A liquid kit uses drops and a color chart, takes a few minutes, and gives readings you can trust. The gap matters most during cycling, when a strip that says zero can hide a dangerous ammonia level. One liquid master kit lasts hundreds of tests and works out cheaper per test than strips over a year. If you already own strips, keep them for a fast daily glance, but confirm anything alarming with a liquid test before you act on it.

During cycling, test every two or three days so you can watch ammonia, nitrite, and nitrate change. With fish in an uncycled tank, test ammonia and nitrite daily. Once the tank is cycled and stable, a weekly test of all four numbers is enough, dropping to every two weeks after a few steady months. Always test when something looks wrong: a fish gasping, sitting on the bottom, or refusing food is a reason to test the same day. Test before and after a big water change while you are troubleshooting. Keep a simple log for the first two months, because a written record shows trends that a single reading hides.

Change the water. A 50 percent change with dechlorinated, temperature-matched water roughly halves the ammonia at once, and you can repeat it. Ammonia above 0 ppm burns gills and is an emergency, not a wait-and-see. After the change, stop feeding for a day or two so you are not adding more waste, and test again in 12 to 24 hours. Do not rinse your filter, do not tear the tank down, and do not reach for ammonia-removing chemicals first, because the water change is faster and safer. Then find the cause: an uncycled tank, overfeeding, a dead fish, or a filter that was cleaned too hard. Fix the cause or the reading comes straight back.

Most tropical community fish do well at 76 to 80°F (24 to 27°C), held steady. Stability matters more than an exact number, because a swing of several degrees in a day stresses fish more than a steady reading at the edge of the range. Use an adjustable, thermostat-controlled heater sized to your tank, and trust a separate thermometer over the dial on the heater. Coldwater fish like goldfish prefer 65 to 72°F (18 to 22°C) and need no heater in a warm room. Check the species you keep, since a few, such as some bettas, want the warmer end near 78 to 80°F (26 to 27°C). Keep the tank out of direct sun, which overheats it.

Yes. Tap water in most areas contains chlorine or chloramine, added to make it safe to drink, and both kill fish and the beneficial bacteria in your filter. A water conditioner, also sold as a dechlorinator, neutralizes them instantly, so add it every time you put tap water in the tank. A few drops per gallon treats a bucket in seconds. Never add untreated tap water to a tank with fish. Well water usually has no chlorine but can carry high iron or low oxygen, so test it. Letting water stand overnight removes chlorine but not chloramine, which is now common, so a conditioner is the reliable choice. Match the new water's temperature as well.

No, treat it as a rough starting point that fails often. The rule says one inch of fish per gallon of water, so a 20 gallon (76 litre) tank could hold twenty inches of fish. The trouble is that it ignores body shape, waste output, swimming space, and behavior. A single 10 inch goldfish produces far more waste than ten 1 inch tetras and needs much more room. Territorial fish need space the math never captures. Use the rule only to spot obvious overstocking, then research the adult size and needs of each species. Filtration, water change frequency, and temperament decide real capacity. When unsure, keep fewer fish, because an understocked tank is easier and healthier.

A 10 gallon (38 litre) tank suits a small group of nano fish, for example six to eight small tetras or rasboras, or one betta with a few snails. Think in adult size and waste, not the size at purchase, because those half-inch fish grow. Skip anything that reaches over 2 inches or needs a large school, and skip goldfish entirely, since one goldfish needs 20 to 30 gallons (76 to 114 litres) on its own. Add fish slowly, a few at a time over weeks, so the filter bacteria keep up with the extra waste. Stock lightly for a first tank. A lightly stocked 10 gallon (38 litre) tank is far more stable than a crowded one.

Check three things: adult size, temperament, and water needs. Fish that want the same temperature and pH range, sit close in size, and share a calm temperament usually live together well. Large mouths eat smaller tankmates, so a fish that will eventually fit in another's mouth is a problem. Fin nippers like some barbs shred long-finned fish such as bettas and fancy guppies. Some species are territorial and fight their own kind or lookalikes. Research each fish before buying, and look up its adult size rather than the juvenile in the store tank. Buy schooling fish in groups of six or more, since a lone schooling fish is stressed and often aggressive. Conflicting needs mean separate tanks.

Acclimate them slowly to your water temperature and chemistry. Float the sealed bag in the tank for 15 minutes so the temperatures match. Then open the bag and add a half cup of tank water every five minutes for 20 to 30 minutes, which lets the fish adjust to your pH and hardness by degrees. Net the fish into the tank and pour the bag water down the drain rather than into the tank, because it can carry disease or ammonia. Keep the lights off for a few hours afterward to lower stress. Do not tip a fish straight from the bag into the tank, and do not rush, because a sudden chemistry change can shock and kill an otherwise healthy fish.

No, add them a few at a time over several weeks. Every fish adds waste, and your filter bacteria need time to grow to match the new load. Dumping a full stock in at once causes an ammonia spike the young colony cannot process, even in a cycled tank. A safe pace is two to four small fish per week in an average community tank, testing ammonia and nitrite between additions. If a reading climbs above 0 ppm, stop adding and change water until it settles. Going slowly also lets you watch new arrivals for illness before they join the whole group. Patience here prevents the crash that can kill a full tank of fish in a weekend.

Feed only what the fish finish in about two minutes, then remove anything left over. A fish's stomach is roughly the size of its eye, so a tiny amount is a full meal. Uneaten food rots and turns into ammonia, which is the main way overfeeding harms fish. For most community fish, a pinch of flake or a few pellets once or twice a day is plenty. Soak dried pellets for a minute first so they swell in the water rather than inside the fish. Underfeeding beats overfeeding, because a healthy adult can go several days without food but cannot survive fouled water. Watch the fish eat, and cut the amount down if food is reaching the substrate.

Once or twice a day for most adult fish, and a healthy adult can skip a day with no harm. Young growing fish and active species may want two or three small meals, while many adults do fine on a single feeding. One fasting day a week rests the digestive system and helps prevent constipation, which is common in overfed fish. Feed at set times so the fish learn the routine and come out to eat, which also lets you confirm everyone is active and healthy. Bottom feeders and nocturnal species like some catfish need sinking food and may feed after lights out. Match the schedule to the fish you keep, and keep each meal small.

Watch for food on the substrate, cloudy water, rising ammonia or nitrate, and a layer of sludge in the gravel. If flakes drift to the bottom uneaten, you fed too much. Overfeeding is the most common beginner mistake, and it harms fish by fouling the water rather than by stuffing them. A swollen belly, stringy waste, or a fish that struggles to stay off the bottom after eating can point to overfeeding too. Cut back to what the fish clear in two minutes, scoop out leftover food with a net, and vacuum the gravel at your next water change. If the water clouded within a day of feeding, that is a bacterial bloom living on the excess, and less food fixes it.

For a trip of up to a week, the safest answer is to not feed them at all. A healthy adult fish handles seven days without food far better than it handles a well-meaning neighbor fouling the water. For longer trips, an automatic feeder set to a small daily portion is more reliable than a person. Avoid the weekend and vacation feeder blocks, because they dissolve and can spike ammonia or cloud the water. If a person will feed for you, pre-portion each day's food into a labeled container so they cannot tip in too much. Do a water change before you leave and check the heater and filter, since equipment failure is a bigger vacation risk than hunger.

Most often it is a bacterial bloom, which is harmless and clears on its own. In a new tank, a white or gray haze in the first few weeks is bacteria multiplying in the water, and it settles once the cycle matures. Do not fight it with large daily water changes, because that just feeds it, and let it run its course while you test ammonia. A green tint is different: that is algae from too much light, often direct sun, and it needs less light rather than time. A cloud right after adding substrate is usually dust that settles or filters out within a day. Match the color and timing to the cause, and test the water to rule out an ammonia problem.

Too much light, too many nutrients, or both. Algae grows when light and nutrients outpace what the plants and system use, so the fix is to cut lighting to 6 to 8 hours a day on a timer and feed less. Direct sunlight is a common cause that no timer solves, so move the tank out of the sun. Overfeeding and skipped water changes raise the nutrients algae feeds on, so feed less and change 25 percent of the water weekly. Remove what is there by hand, scrape the glass, and add fast-growing or floating plants that outcompete algae for nutrients. A little algae is normal and harmless. A sudden outbreak points to a light or feeding change you can reverse.

Gasping means the fish cannot get enough oxygen, and it has two main causes. First, a water quality problem: ammonia or nitrite burns the gills so the fish cannot breathe even in oxygen-rich water, so test at once and change 50 percent of the water if either reads above 0 ppm. Second, low dissolved oxygen from warm water, crowding, or poor surface movement, since warm water holds less oxygen. Add surface agitation by lowering the water line under the filter outflow or running an air stone. Check the temperature, because a tank over 82°F (28°C) sheds oxygen fast. If one fish gasps while others act normally, it may be sick or bullied. If all of them gasp, treat it as an emergency and test now.

Small white spots like grains of salt scattered over the body and fins are almost certainly ich, a common parasite, especially if the fish also flicks against decor. Ich takes hold when fish are stressed or after a temperature drop. Treat it by raising the temperature slowly to about 80°F (27°C) if your fish tolerate it, which speeds the parasite's life cycle so treatment can reach it, then follow a proper ich medication or aquarium salt routine. Do not ignore it, because ich spreads fast and can empty a tank. Fuzzy white cottony patches are a different problem and point to a fungal or bacterial infection. Whatever the spots, test the water first, since poor conditions trigger most outbreaks.

Remove the body promptly, because a decaying fish releases ammonia, fouls the water, and can spread disease. Then test the water for ammonia, nitrite, nitrate, and pH, since a bad reading is the most common cause of a sudden death and it threatens the survivors too. Look the dead fish over for ich, fungus, or injury that might hint at a cause. If the water tests clean and only one fish died with no marks, it may simply have been old or weak, and that does happen. If several die in a short span or the water tests badly, treat it as an emergency, change 50 percent of the water, and hold off on adding new fish until you find and fix the cause.