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Fin & Filter

Answers

Frequently asked questions

The twenty questions new fishkeepers ask most, answered as briefly as they can honestly be answered, each linking to the page that works it through properly.

If you read one answer on this page, read the first one. Almost every first-tank disaster traces back to the same thing: fish went in before the filter had grown the bacteria that make the water survivable.

These are the questions that come up most often, answered as briefly as they can be answered honestly. Each links to the page that works it through properly.

Questions people actually ask

Frequently asked questions

+ Why do my fish keep dying in a new tank?
Almost always because the tank was not cycled. A brand-new filter contains none of the bacteria that convert fish waste into something harmless, so ammonia accumulates from the first day and burns the gills of everything in the water. It is invisible and odorless, which is why it catches people out. The fix is to grow those bacteria BEFORE adding fish, which takes about four to six weeks. Full method on how to cycle a fish tank, and the emergency version if fish are already in there on why do my fish keep dying.
+ How long does it take to cycle a fish tank?
Four to six weeks without a bacterial starter, and often two to three with a good one. It is finished when ammonia reads 0 ppm and nitrite reads 0 ppm within 24 hours of dosing ammonia, with nitrate present. Not before — a tank that has been running two weeks and looks clear is usually mid-cycle, and clear water tells you nothing about ammonia. See how long does it take to cycle a fish tank.
+ Can I speed up cycling a tank?
Genuinely yes, in one way: seed it with established bacteria. Filter media, a sponge or gravel from a healthy running tank is the fastest method, and a bottled starter is the next best. What does not work is water changes, adding more fish, more food, or waiting for the water to look clear. Detail on bacteria starters.
+ How many fish can I put in a 10 gallon tank?
Fewer than the old one-inch-per-gallon rule suggests, because that rule ignores adult size, body mass and waste output. For a 10 gallon tank a realistic answer is one betta, or a single small school of a nano species, and not both. Worked examples on how many fish in a 10 gallon tank.
+ Do I need a filter for a betta?
Yes — with a caveat about flow rather than about whether to have one at all. Bettas are labyrinth fish and can breathe air, which is where the myth comes from, but they still live in the water and that water still needs its ammonia processed. What they cannot tolerate is strong current. A gentle sponge filter, or a hang-on-back filter baffled down, is the answer. See do I need a filter for a betta.
+ What size aquarium filter do I need?
Take your tank volume, multiply by 6 for a typical community tank to get the real flow you need, then divide by 0.7 to get the RATED figure to shop for — because a filter loses roughly 30% of its rated flow to media and head height in service. A 10 gallon tank therefore wants a filter rated 85 GPH or more, not 60. Full method on how to size an aquarium filter.
+ Do I need a heater for my fish tank?
It depends on the species and on your coldest room temperature, not on the tank size alone. Tropical community fish want stable warmth, and a room that drops several degrees overnight stresses them even when the daytime reading looks fine. Wattage depends on the lift you need: about 5 watts per gallon for a 10 °F lift, 10 watts per gallon for a 20 °F lift. See best aquarium heaters.
+ What does a starter kit leave out?
Consistently: a test kit (no kit we examined includes one), a usable quantity of dechlorinator rather than a sample sachet, and substrate. Often also an adjustable heater — several kits ship a preset unit you cannot dial. Kit by kit on what starter kits leave out.
+ Are test strips good enough?
For a quick recheck on an established tank, yes. For watching a cycle, no — and there is a specific reason. Strips read in bands, and the difference that matters most during cycling is between 0 and 0.25 ppm, which is exactly where a banded reading is weakest. Worse, the common six-in-one strips do not test ammonia at all. Comparison on test strips vs liquid test kits.
+ Why is my fish tank water cloudy?
If it is milky white and appeared a day or two after setup, it is a bacterial bloom, which is normal and clears on its own — do not fight it. If it is green, that is algae and a light problem. If it is brown or hazy, it is usually unrinsed substrate or fine debris. The full diagnostic is on why is my fish tank cloudy.
+ How often should I do a water change?
About 25% weekly for a typical stocked community tank, using dechlorinated water matched to tank temperature. More during a fish-in cycle, where the water change is the only thing keeping ammonia survivable. Less is a slow accumulation of nitrate. See how often to change aquarium water.
+ Do I need a water conditioner?
If you use tap water, yes, every time. Municipal water is treated with chlorine or chloramine, both of which are lethal to fish and to the bacteria in your filter. Chloramine in particular does not gas off if you let water stand — that advice predates chloramine and is now wrong in most of the US. See best aquarium water conditioners.
+ Can I use bottled or distilled water instead?
Distilled water is a bad idea on its own: it has been stripped of the minerals fish need and of the buffering that keeps pH stable, so a distilled-water tank swings dangerously. Treated tap water is better for almost everyone. Bottled drinking water varies too much between brands to rely on.
+ How many fish can I add at once?
In a newly cycled tank, a few small fish, then wait two weeks and test. The bacterial colony sizes itself to the waste load it is fed, so a large addition outruns it and produces an ammonia spike in an otherwise cycled tank. See adding fish to a new tank.
+ Can a goldfish live in a 10 gallon tank?
No, and this is one of the most consequential myths in the hobby. Common goldfish reach a foot or more and are heavy waste producers; even fancy varieties need far more water than a 10 gallon tank provides. They also do not stay small in a small tank — they become stunted, which is a deformity, not an adaptation. See what size tank do goldfish need.
+ What are the hardiest beginner fish?
Species that tolerate a range of conditions and are not fragile in transit: certain small tetras, danios, platies, corydoras in groups, and a single betta in a properly heated tank. Hardy is not the same as invincible — every one of them dies in an uncycled tank. See hardiest beginner fish.
+ Do live plants help?
They help, but they are not a filter. Plants consume nitrate and some ammonia and improve stability, but they will not carry the ammonia load of a stocked tank, and dying plants add to it. Easy low-light species are worth having; they are not a substitute for cycling. See easy live plants for beginners.
+ Do I have to buy a whole starter kit, or can I buy separately?
Either works. A kit is usually cheaper and guarantees the hood, light and filter fit the tank, which is worth something on a first build. Buying separately lets you avoid the two common kit compromises — a preset heater and an undersized filter. The comparison is on best aquarium starter kits.
+ Have you tested the products you recommend?
No, and we say so on every page. There is no test lab here. Every recommendation comes from published manufacturer specifications and the arithmetic on how we pick, which is printed in full so you can re-run it. Where a manufacturer does not publish a figure, the page says so rather than estimating one.
+ How does this site make money?
Amazon Associates commissions on links you choose to use. You pay the same price either way, no brand pays for placement, and every roundup names at least one product not to buy first — including products we would earn on. Full detail on the affiliate disclosure.