Guide
Fishkeeping fundamentals for new aquarists
A simple framework to build stable tanks, avoid common mistakes, and stay consistent.
- Tank size matters more than you think: A 60-100 litre (15-25 gallon) tank is far more stable and forgiving than a nano setup. Start there if you can.
- Never skip cycling: Run the nitrogen cycle before adding fish. This takes 4-6 weeks and is the single biggest factor in whether your fish survive the first month.
- Test your water, don't guess: Ammonia and nitrite should read 0 ppm at all times. Nitrate should stay below 20-40 ppm between water changes.
- Stock slowly and research first: Check adult size, social needs, temperature overlap, and pH compatibility before you buy anything. Impulse purchases cause most beginner losses.
- Weekly maintenance beats monthly rescues: A 25-30% water change each week, a quick filter check, and a visual health scan take 20 minutes and prevent most problems.
- Fish are not low-maintenance pets: The equipment is cheap. The attention is not optional. Be honest with yourself about the time commitment before you start.
Choose the right tank size from the start
The most common piece of advice new fishkeepers receive is "start small." It sounds logical - smaller tank, smaller commitment. In practice, it's wrong. A 20-litre (5-gallon) tank swings from safe to toxic faster than you can react. Temperature spikes, ammonia builds, and pH crashes happen over hours in small volumes of water. In a 80-litre (20-gallon) tank, the same mistakes give you time to fix things before fish die.
A practical starting point for most beginners is 60-100 litres (15-25 gallons). It's large enough to house a proper community, stable enough to handle beginner mistakes, and small enough to fit in most living rooms without requiring structural reinforcement.
You'll need four pieces of equipment that actually matter: a filter rated for your tank volume (ideally 1.5-2× the tank size in turnover per hour), a heater appropriate to your room temperature and fish species, a thermometer you check daily, and a test kit - liquid reagent, not strips, for accuracy. Everything else - the special substrate, the UV steriliser, the CO2 system - comes later, if at all.
Most starter kits sold at pet shops include filters that are underpowered and heaters with no temperature display. Check the filter's flow rate before you trust it, and buy a separate thermometer regardless of what came in the box.
The nitrogen cycle is not optional
This is where most beginners lose fish, and it's almost entirely preventable. When fish produce waste, it breaks down into ammonia - which is acutely toxic even at 0.25 ppm. Beneficial bacteria in your filter convert ammonia to nitrite (also toxic), then nitrite to nitrate (far less harmful in low concentrations). That bacterial colony doesn't exist in a new tank. You have to grow it first.
A full fishless cycle typically takes 4-6 weeks. You add an ammonia source (pure ammonia solution or fish food), let it build to around 2-4 ppm, and wait for the bacterial colony to establish and process it down to 0. Once ammonia and nitrite both read 0 ppm within 24 hours of dosing, the tank is cycled and ready for fish.
For a detailed walkthrough of the process, read the guide on how to cycle a fish tank and the supporting piece on the nitrogen cycle explained.
Adding fish to a new tank after 24-48 hours because "the water looks clear." Clear water tells you nothing. Only a test kit tells you whether ammonia and nitrite are at lethal levels. New tank syndrome kills more fish than disease.
Rushing this with a full stocking almost always results in ammonia or nitrite spikes that stress or kill fish within the first month.
Water quality: what to test and what the numbers mean
Once your tank is running, water chemistry becomes your ongoing responsibility. The parameters that matter most for a freshwater community tank are:
- Ammonia (NH₃/NH₄⁺): 0 ppm always. Any detectable ammonia in a cycled tank means something is wrong - overfeeding, a dead fish, a filter failure, or overstocking.
- Nitrite (NO₂⁻): 0 ppm always. Like ammonia, any reading above zero in an established tank is a warning sign.
- Nitrate (NO₃⁻): Below 20 ppm for sensitive species, below 40 ppm as a general ceiling. Controlled primarily by regular water changes.
- pH: Depends on your fish. Most community species do well between 6.8-7.6. Stability matters more than hitting an exact target - a steady pH of 7.4 is better than one that swings between 6.8 and 7.8.
- Temperature: Most tropical freshwater fish need 24-27°C (75-81°F). Check this daily with a thermometer - heaters fail.
- KH (carbonate hardness): Often overlooked. KH buffers pH against sudden crashes. Aim for at least 4-5 dKH to keep pH stable.
For a full breakdown of ranges and what affects each parameter, see the guide on aquarium water parameters.
Test weekly and log every reading. You're looking for trends, not just single data points. A nitrate that rises from 10 to 30 ppm between water changes tells you more than a single reading of 20 ppm. App-aquatic's parameter tracking does this automatically so you can spot drift before it becomes a problem.
Water changes: the most underrated maintenance task
A working filter handles ammonia and nitrite. It does not remove nitrate, dissolved organic compounds, or minerals that accumulate over time. Water changes do. There is no equipment substitute for regular partial water changes - not a bigger filter, not a protein skimmer, not a UV steriliser.
For most community tanks, 25-30% weekly is the standard. Heavily stocked tanks or tanks with large, messy fish (goldfish, cichlids) may need 30-50% twice a week. Lightly stocked, heavily planted tanks can sometimes go to 10-15% weekly, but that requires healthy plant growth actually consuming nitrate.
Always dechlorinate replacement water with a water conditioner before adding it. Match the temperature as closely as possible - pouring cold water into a warm tank stresses fish and can trigger disease outbreaks. Aim within 1-2°C of the tank temperature.
For more detail on scheduling and technique, see how often to do water changes.
Don't clean your filter media under the tap. Chlorinated tap water kills the beneficial bacteria that make your filter work. Rinse filter sponges and media in a bucket of tank water removed during a water change, and only when flow visibly drops - not on a fixed calendar schedule.
Stocking: research before you buy, every time
Fish compatibility isn't just about aggression. You need to check four things for every species you're considering:
- Adult size: That 3 cm fish in the shop may reach 15 cm. A common pleco (Hypostomus plecostomus) sold as a "tank cleaner" at 5 cm will grow to 45-60 cm. Know what you're buying.
- Social needs: Many fish are shoaling species that need groups to feel secure. Neon tetras (Paracheirodon innesi) kept in groups of three show chronic stress. Keep them in groups of eight or more.
- Temperature and pH overlap: Mixing discus (28-30°C, soft acidic water) with goldfish (18-22°C, hard alkaline water) doesn't work. Build your stocking list around a defined water profile, then find fish that fit it.
- Bioload: Some fish produce far more waste than their size suggests. Goldfish, oscars, and large cichlids require significantly more filtration and water change frequency than equivalent-sized community fish.
Add fish in small groups with two to three weeks between additions. This gives your filter bacteria time to adjust to the increased bioload and gives you time to spot problems with new arrivals before they affect your existing fish.
For species-specific guidance, read the guide on best fish for beginners, or run your planned stocking list through the free stocking calculator to check compatibility and tank capacity.
The "inch per gallon" stocking rule is not useful. It treats a 10-inch oscar the same as ten 1-inch neon tetras, ignores bioload differences between species, and doesn't account for water column usage or territorial behaviour. Use it only as a rough ceiling - never as a green light.
Feeding: less than you think, more often than you do
Overfeeding is one of the most direct causes of water quality problems. Uneaten food decomposes, drives ammonia up, and creates a cycle of water changes and stress that's entirely avoidable.
Feed once or twice daily, with only what fish consume in 2-3 minutes. Remove any uneaten food with a siphon or net after feeding. Fast your fish one day per week - it reduces waste load and mimics natural feeding patterns without causing harm to healthy adult fish.
Vary the diet. Dry flake or pellet as a base is fine, but most fish benefit from occasional frozen or live food (bloodworm, daphnia, brine shrimp). This supports immune function and keeps fish in better condition long term.
Maintenance habits that actually work
Most tank problems don't appear overnight. They develop gradually - a filter slowing, a heater drifting, nitrate creeping up, a fish losing colour. The way to catch them is consistent, brief, regular attention rather than occasional deep-cleans.
A practical weekly routine for most community tanks looks like this:
- Check temperature and fish behaviour during morning feeding
- Test ammonia, nitrite, and nitrate (weekly minimum)
- Perform a 25-30% water change
- Wipe algae from the front glass if needed
- Check filter flow rate - a drop in output usually means the sponge needs rinsing
- Trim any dead plant material
Monthly, check your heater's set point against your thermometer (they drift), inspect hoses and seals for wear, and clean the impeller on your filter. These checks take minutes and prevent equipment failures that can wipe out a tank overnight.
Set recurring reminders for water changes and filter checks. Missed maintenance is almost always a scheduling issue, not a knowledge one. A simple reminder system - whether in a calendar app or a fishkeeping app - makes consistency dramatically easier to sustain.
What most beginner guides miss
A few things experienced fishkeepers know that rarely appear in starter articles:
Quarantine new fish. A separate 20-40 litre quarantine tank is one of the best investments you'll make. Run new fish through two to four weeks of quarantine before they enter your display tank. Most disease introductions - ich, velvet, bacterial infections - come in on new fish that looked healthy in the shop.
Your tap water profile determines your fish options, not the other way around. Test your tap water before you decide what to keep. Very soft, acidic water (below 6.5 pH, under 3 dKH) is difficult to buffer and suits soft-water species. Hard, alkaline water (above 7.8 pH, over 15 dKH) suits livebearers and rift lake cichlids but is poor for soft-water species like discus or wild bettas. Work with your water, not against it.
A heater failure kills fish faster than almost anything else. Heaters fail both ways - they stop heating and they stick on. A thermometer with a min/max memory, or a smart heater controller, can alert you to a failure before it becomes fatal. Check temperature every morning as a habit.
Plants genuinely help. Live plants consume ammonia and nitrate, outcompete algae for nutrients, and provide cover that reduces fish stress. You don't need a planted tank setup - easy species like Vallisneria, Anubias, or Ceratophyllum grow in most conditions with no specialist equipment.
Track your parameters and get care reminders with App-aquatic.
Get the free appHow long does it take to set up a fish tank before adding fish?
Allow 4-6 weeks minimum for a fishless nitrogen cycle. The tank and equipment can be set up in a day, but the biological filtration that keeps fish alive takes weeks to establish. Testing ammonia and nitrite to confirm they've both reached 0 ppm is how you know it's ready - not the calendar.
How many fish can I put in my tank?
It depends on the species, not just the volume. Bioload, adult size, and territorial behaviour all affect how many fish a tank can realistically support. A rough guide is 1 cm of adult fish body length per 2-3 litres, but this has real limitations. Use the stocking calculator for a more accurate assessment, and always stock gradually.
Why are my fish dying even though the water looks clean?
Clear water can still contain lethal levels of ammonia or nitrite, especially in a tank that hasn't fully cycled. Test your water immediately - ammonia above 0.5 ppm or any detectable nitrite requires an emergency partial water change. Also check temperature, ensure your filter is running, and look for any dead fish or decaying food that may be spiking ammonia.