Guide
What should I get as my first tank?
Counter-intuitive but true: sometimes it's better to go bigger for your first tank. Here's why a larger aquarium can be easier to manage.
- Bigger is more forgiving: A 20-30 gallon (75-115 L) tank is the sweet spot for most beginners - enough water volume to stay chemically stable, small enough to fit most rooms and budgets.
- Small tanks are not beginner tanks: In 5 gallons, a single overfeeding event or skipped water change can spike ammonia to lethal levels within hours.
- Cycle before any fish go in: Running the nitrogen cycle first - typically 4-6 weeks - is the single biggest factor in whether your first fish survive.
- Filter for more than the listed tank size: A filter rated for 20 gallons on a 20-gallon tank is borderline. Size up.
- Stock lightly and slowly: Add fish in small groups over weeks, not all at once on day one.
- Test your water, don't guess: A liquid test kit (API Master Test Kit or similar) gives you real numbers; test strips are better than nothing but less accurate.
The "small tank equals easy" myth
This is the most damaging piece of advice in the hobby, and it usually comes from pet store staff trying to make a sale. The logic sounds reasonable: less water, less equipment, less money. What they don't tell you is that less water also means less stability - and less time to fix your mistakes before fish start dying.
Water chemistry in a small tank can shift dramatically in a matter of hours. In a 5-gallon tank, one dead fish decomposing overnight can push ammonia from 0 to toxic levels before you notice anything is wrong. In a 30-gallon tank, the same event barely registers. That buffer - that extra time to catch problems - is what actually makes fishkeeping manageable for a beginner.
Nano tanks (under 10 gallons) have their place in the hobby. Experienced keepers run beautiful planted 5-gallon betta tanks. But experienced keepers understand cycling, know their parameters cold, and do frequent maintenance. That knowledge takes time to build. You want a tank that gives you room to learn while you're building it.
Buying a "starter kit" under 10 gallons because it's cheaper upfront. These kits often come with underpowered filters and no heater. The fish loss that follows - and the replacement fish, treatments, and heartbreak - ends up costing more than a proper setup from the start.
Why 20-30 gallons is the right starting point for most people
There's nothing magic about the number, but a 20-30 gallon tank (roughly 75-115 litres) consistently outperforms smaller options for beginners for a few concrete reasons.
Temperature stays stable
Tropical fish need water held between 24-28°C (75-82°F) depending on species. A small tank heats and cools faster with ambient temperature swings - an open window in summer or a cold night in winter can shift a 5-gallon by 3-4°C in hours. A 30-gallon has enough thermal mass to absorb those changes gradually, giving your heater time to compensate.
The nitrogen cycle establishes more reliably
Beneficial bacteria colonies - the ones that process fish waste from toxic ammonia into less harmful nitrate - take time to grow. In a larger volume with stable conditions, they establish more predictably. Tiny tanks are prone to mini-cycles (sudden ammonia spikes) even after the initial cycle completes, because any disruption - a filter clean, a new fish, a medication dose - can destabilise a colony that never had much room to grow.
You have actual stocking options
In 5 gallons, your realistic options are one betta or a handful of small shrimp. In 20-30 gallons, you can keep a school of ember tetras (Hyphessobrycon amandae), a group of corydoras, a small centrepiece fish, and still have room to breathe. Fishkeeping becomes interesting when you have choices. Check the free stocking calculator before you commit to any combination.
This is how long it typically takes for beneficial bacteria to establish before the tank is safe for fish. Rushing this is the number one cause of early fish deaths.
Cycling: the step most beginners skip entirely
Set up a new tank, let it run for 24 hours, add fish. That's what most beginners do, and it's why so many first tanks end in disaster within the first month. A brand-new tank has no beneficial bacteria to process fish waste. Ammonia builds up immediately, poisons the fish, and the keeper usually has no idea what went wrong.
The nitrogen cycle needs to complete before fish go in. Ammonia (from fish waste and uneaten food) gets converted to nitrite by one group of bacteria, then nitrite gets converted to less harmful nitrate by another group. Both nitrite and ammonia are acutely toxic to fish even at low concentrations. Nitrate builds up over time and gets removed through water changes.
To cycle your tank before adding fish, you need an ammonia source. Options include pure ammonia (dosed carefully), fish food left to decompose, or - if you must - a single very hardy fish like a zebra danio (Danio rerio). The fishless method is better because no animals are stressed in the process. The cycle is complete when you can add ammonia to 2-3 ppm and have it read zero within 24 hours, with nitrite also at zero.
Seed your new filter with media from an established tank - a handful of gravel, a piece of sponge, or a used filter pad from a friend's healthy aquarium. This jumpstarts your cycle by introducing existing bacterial colonies. A cycled tank that would normally take 6 weeks can sometimes be ready in 10-14 days this way.
Filtration: size up, not down
Most filter packaging lists a maximum tank size on the box. Treat that number as the minimum tank it'll work adequately on, not the ideal. A filter rated for "up to 20 gallons" running on a 20-gallon tank stocked with fish is working at full capacity with no headroom. If you overstock slightly, miss a maintenance session, or your bioload increases for any reason, it tips over.
For a 20-30 gallon tank, look for a filter rated to 40-50 gallons. This gives you genuine biological filtration capacity, better flow, and a margin for error. The two common types for beginner tanks are hang-on-back (HOB) filters and internal canister filters. Both work. HOB filters are easier to maintain and have the media more accessible; internal filters are quieter and easier to hide in the tank.
What the filter media is matters too. Sponge or ceramic bio-media provides surface area for beneficial bacteria. Carbon is often included in starter kits but only removes dissolved organics temporarily - it exhausts within a few weeks and doesn't replace proper biological filtration. Don't rely on it as your primary media.
Never clean all your filter media at once, and never clean it under tap water. Chlorine in tap water kills beneficial bacteria. Rinse filter sponges and ceramics in a bucket of tank water removed during a water change - this cleans them without destroying the colony.
What your water parameters actually need to be
Before you buy fish, know your tap water. pH, hardness (GH and KH), and temperature out of the tap vary enormously by location, and they determine which fish you can realistically keep without constantly fighting your water chemistry.
Most common beginner fish - neon tetras (Paracheirodon innesi), platies (Xiphophorus maculatus), corydoras species, guppies (Poecilia reticulata) - are adaptable to a range of conditions. But there's a difference between adaptable and thriving. Check the full parameter guide to understand what your tap water's numbers mean for stocking decisions.
The parameters you need to test for in a new tank:
- Ammonia: Should be 0 ppm once cycled. Anything above 0.25 ppm is stressful; above 1 ppm is dangerous.
- Nitrite: Should be 0 ppm once cycled. As toxic as ammonia in the short term.
- Nitrate: Acceptable up to 20-40 ppm for most fish. Kept down by regular water changes.
- pH: Aim for stability over perfection. A stable pH of 7.4 causes less damage than a pH that fluctuates between 6.8 and 7.6 week to week.
Stocking: slower than you think, lighter than you expect
The old "one inch of fish per gallon" rule is useless. A 10-inch oscar and ten 1-inch tetras are not equivalent bioloads. What matters is adult size, waste production, activity level, and compatibility - not a simple length measurement.
For a 20-gallon first tank, a sensible beginner stocking might look like this: a school of 8-10 small tetras or rasboras, 4-6 corydoras for the bottom, and optionally one small centrepiece fish. That's a full, interesting tank that won't overwhelm your filter or create territorial problems. Check the best beginner fish guide for species that genuinely tolerate new-keeper mistakes.
Add fish in stages, not all at once. Add your first group, wait 2-3 weeks, test your parameters, confirm the cycle is holding, then add the next group. Each addition increases the bioload on your filter. Giving the bacterial colony time to grow between additions prevents ammonia spikes.
Every beginner wants to fill their tank immediately. Every experienced keeper wishes they'd stocked slower. There's no shortcut here that doesn't carry real risk to the animals. A half-stocked tank for a few extra weeks is far better than a crashed tank you have to restart from scratch.
Maintenance: what you actually need to do weekly
A healthy aquarium isn't maintenance-free, but the workload is manageable if you set up correctly. The core routine for a 20-30 gallon tank:
- Weekly water change: 25-30% of tank volume, replaced with dechlorinated water matched to the tank temperature within 1-2°C. This is the single most effective thing you can do for fish health. See the water change guide for specifics.
- Weekly parameter test: Especially in the first 3 months. Test ammonia, nitrite, and nitrate every week until you're confident the tank is stable.
- Glass clean: Wipe algae from the front glass with a magnetic cleaner or algae scraper before water changes, so the debris gets removed with the water change.
- Filter check: Monthly. Squeeze sponges in old tank water. Don't scrub them clean - you're just removing the worst debris, not sterilising.
App-aquatic lets you log water test results and set care reminders so your maintenance schedule doesn't slip - useful when you're building the habit in the first few months.
Track your parameters and get care reminders with App-aquatic.
Get the free appCan I start with a 10-gallon tank instead of 20?
You can, but understand the trade-offs. A 10-gallon is much less stable than a 20, limits your stocking to very small fish (think ember tetras or a single betta - not goldfish, not angels), and punishes mistakes faster. If 20 gallons genuinely isn't possible for space or budget reasons, a 10-gallon with a single betta and live plants is a better choice than a heavily stocked nano community tank.
How long do I have to wait before adding fish?
Until the nitrogen cycle is complete - typically 4-6 weeks for a fishless cycle, sometimes faster if you seed with established media. You'll know it's done when ammonia reads 0 ppm and nitrite reads 0 ppm after dosing ammonia to 2-3 ppm. Don't go by days on a calendar; go by what your test kit tells you.
Do I need a heater for my first tank?
Almost certainly yes. Unless you're keeping coldwater fish like white cloud mountain minnows (Tanichthys albonubes) or goldfish, your fish need stable tropical temperatures between 24-27°C (75-81°F). Room temperature in most homes fluctuates too much without a thermostat heater. Buy an adjustable heater and a separate thermometer to verify it's holding temperature correctly - heaters can fail stuck-on as well as stuck-off.
What's the cheapest way to set up a first tank properly?
Buy secondhand. A used tank, filter, and heater from a fellow hobbyist or Facebook Marketplace is often half the price of new and just as functional. The one thing worth buying new is filter media (you want to know what you're starting with) and a liquid test kit. Substrate, decorations, and equipment are all fine used. Just inspect for cracks and test the heater and filter before committing.
Can I keep goldfish as my first fish?
Goldfish are a poor choice for a first 20-30 gallon tank. A single fancy goldfish needs at least 20 gallons on its own and produces waste volume that overwhelms beginner setups fast. Common goldfish are pond fish that can hit 30 cm (12 inches). They're not cruel to keep - but they need either a large tank (55+ gallons for a pair of fancies) or a pond, strong filtration, and frequent water changes. Start with tropical community fish and come back to goldfish once you understand the maintenance demands.
More guides · How to cycle · Overstocking · Maintenance schedule · App features