Guide
Tropical fish keeping tips
Simple habits that keep tropical tanks stable and fish healthy.
- Cycle first, stock second: never add fish to an uncycled tank - ammonia spikes kill fish within days. A full nitrogen cycle takes 4-6 weeks.
- Temperature stability matters more than the exact number: most tropical fish thrive between 24-27°C (75-81°F), but swings of even 2°C in 24 hours cause stress and disease.
- Stock slowly: adding too many fish at once overwhelms your biological filter. Add in small groups, then wait 2-3 weeks before adding more.
- Feed less than you think: one small feeding per day - what fish clear in 2 minutes - is enough. Uneaten food is the fastest route to an ammonia problem.
- Test, don't guess: ammonia and nitrite should read 0 ppm in an established tank. Nitrate should stay below 20-40 ppm with regular water changes.
- Routine beats reaction: a weekly 25-30% water change and a monthly filter rinse will prevent most problems before they start.
Get the cycle right before anything else
Every piece of tropical fishkeeping advice ultimately comes back to one thing: the nitrogen cycle. Beneficial bacteria in your filter convert toxic ammonia (fish waste) into nitrite, then into the far less harmful nitrate. Until that bacterial colony is established, your tank water is dangerous - even if it looks perfectly clear.
The problem is that a new tank has no bacteria at all. Most beginners buy a tank on Saturday, fill it on Sunday, and add fish on Monday. Those fish then sit in water where ammonia climbs unchecked. They may survive a week. They may look fine for two. Then they start dying, and the owner assumes they bought sick fish.
The fix is to cycle the tank before stocking it. Run the filter with an ammonia source - pure ammonia drops, a few fish food flakes, or a piece of raw prawn - for 4-6 weeks. Test ammonia and nitrite every few days. When both read 0 ppm and nitrate is rising, your cycle is complete.
Speed up cycling by seeding your new filter with a handful of gravel or a sponge from an established tank. You're transferring the bacteria that would otherwise take weeks to grow from scratch. A 4-week cycle can sometimes become a 10-day cycle this way.
Temperature: stable first, correct second
Most tropical fish species kept in the hobby do well at 24-27°C (75-81°F), but the specific range matters less than consistency. A tank that sits at 26°C (79°F) year-round is far healthier than one that drifts between 23°C and 28°C with the seasons.
Rapid temperature change stresses fish and suppresses their immune systems - which is exactly when ich (Ichthyophthirius multifiliis) and other opportunistic parasites take hold. Check your heater regularly, especially in summer when room temperature rises and a heater calibrated for winter may overshoot.
A few species-specific notes worth knowing: Discus prefer 28-30°C (82-86°F) and are genuinely demanding. Betta splendens thrive at 26-28°C (79-82°F) and deteriorate noticeably below 24°C. Danios and white cloud mountain minnows tolerate much cooler water - down to 16°C (61°F) - which makes them poor tankmates for warmth-loving species despite being commonly sold together.
Cheap stick-on thermometers read the glass temperature, not the water. They're regularly 1-2°C off. Use a glass or digital probe thermometer placed mid-water for an accurate reading.
Stocking: the rule everyone ignores
The old "1 inch of fish per gallon" rule is useless. It treats a 10-inch common pleco the same as ten 1-inch neon tetras, ignores bioload, and says nothing about swimming space or behaviour. Ignore it.
What actually matters: the adult size of your fish, their bioload (messy fish like goldfish and oscars produce far more waste than their body size suggests), their behaviour (territorial cichlids need more space than schooling tetras), and how fast your filter can process waste.
A practical starting point: stock lightly, observe your water parameters over 2-3 weeks, and only add more fish when ammonia and nitrite are still reading 0 ppm. Use a stocking calculator to cross-check numbers before you buy.
Stocking a tank based on how fish look in the shop, not how big they'll grow. A 5 cm juvenile oscar will reach 30-35 cm (12-14 inches) and outgrow a 200-litre tank. Always research adult size before you buy - not after.
For genuinely beginner-friendly species that are hardy, small, and forgiving of imperfect parameters, the best fish for beginners guide is worth reading before your first purchase.
Feeding: less is almost always more
Overfeeding is one of the most reliable ways to crash water quality in a healthy tank. Uneaten food breaks down, spikes ammonia, feeds algae, and clouds the water. Most healthy adult tropical fish can go two or three days without food without any ill effect - their wild counterparts frequently do.
A practical approach: feed once a day, an amount your fish clear in under two minutes. If there's food sitting on the substrate 10 minutes after feeding, you're feeding too much. Vary the diet - quality flake or pellet as a base, with occasional frozen or live food (bloodworm, daphnia, brine shrimp) to provide nutrients that dry food doesn't fully replicate.
Fasting your fish one day a week is a genuinely useful habit. It reduces waste output, gives your filter breathing room, and has no negative effect on fish health.
If your fish haven't eaten everything within two minutes, you're adding too much food - and the excess is heading straight into your nitrogen cycle as ammonia.
Water parameters: what to test and when
The six parameters worth tracking in a tropical freshwater tank are ammonia, nitrite, nitrate, pH, hardness (GH/KH), and temperature. In a cycled, healthy tank you're mainly watching nitrate and pH - the others should be stable once your cycle is established.
For a detailed breakdown of target ranges and what each number means, see the aquarium water parameters guide. The short version: ammonia and nitrite should always be 0 ppm. Nitrate below 40 ppm for most fish, below 20 ppm for sensitive species. pH between 6.5 and 7.5 covers the majority of commonly kept tropical species, though there are meaningful exceptions - African Rift Lake cichlids want pH 7.8-8.5, while many South American species like cardinal tetras prefer 5.5-6.5.
The mistake most people make isn't testing the wrong things - it's testing reactively. They test when fish look ill. By that point, parameters may have been bad for weeks. Test on a schedule: weekly for the first 6 months of a new tank, then at least fortnightly once things are stable. Log the results somewhere - a notebook, a spreadsheet, or an app like App-aquatic - so you can spot trends rather than just snapshots.
Liquid test kits (API Master Test Kit is the standard recommendation) are meaningfully more accurate than test strips. Strips are fine for a quick check, but they degrade once the tube is open and frequently misread nitrite. If you're troubleshooting sick fish or an ammonia problem, use liquid tests.
Water changes: the single most effective maintenance task
Nothing in fishkeeping replaces water changes. Filters process ammonia and nitrite, but they don't remove nitrate - only dilution does. Water changes also replenish minerals and carbonate hardness (KH) that buffer your pH, which gradually depletes as fish respire and organic matter breaks down.
The standard advice of 10-15% weekly is a minimum, not an optimum. For heavily stocked tanks, messy fish, or tanks with live plants that benefit from fresh nutrients, 25-30% weekly produces noticeably better fish health. For more detail on scheduling and how tank size affects frequency, the water change frequency guide covers it properly.
Always dechlorinate tap water before adding it to the tank. Match the temperature of new water to within 1-2°C of your tank water before adding it - cold water shocks fish and is a reliable trigger for ich outbreaks.
When you do your weekly water change, use the gravel vacuum on a different section of the substrate each time. Rotate quarters: front left, front right, back left, back right. You'll clear mulm without disrupting the entire bed at once, and you won't have to do the whole thing in a single session.
Filter maintenance: clean it, but not too clean
Your filter media is where your beneficial bacteria live. Most of the fishkeeping disaster stories that start with "I did a big clean and then all my fish died" come from people who cleaned their filter too thoroughly - rinsing media under tap water and killing the bacteria that make the tank safe.
Rinse filter sponges in a bucket of water taken from the tank itself (never tap water), and only clean part of the filter at a time. Replace mechanical media like filter floss on its own schedule without touching the biological media at the same time. A filter that's slightly dirty is doing its job - a filter that's running slowly because it's completely blocked needs attention, but gentle attention.
What most guides skip: pH stability vs. pH perfection
There's a widespread fixation in the hobby on hitting an exact pH for your fish. The reality is that most tropical fish adapt well to pH values across a fairly wide range, as long as that pH is stable. A tank sitting at pH 7.4 that never moves is better for almost any fish than a tank swinging between 6.8 and 7.6 daily.
Daily pH swings happen most often in tanks with live plants and high CO2, or in low-KH tanks where there's insufficient buffering capacity. If your pH is dropping overnight or between water changes, test your KH. Below 4 dKH, you're at risk of pH crashes. Crushed coral or a small amount of sodium bicarbonate will raise KH and stabilise pH - but address the cause, not just the number.
Track your parameters and get care reminders with App-aquatic.
Get the free appHow long should I run my tank before adding fish?
At minimum, until your tank has fully cycled - typically 4-6 weeks. You need to see ammonia and nitrite both reading 0 ppm on consecutive tests before fish are safe. Adding fish before that point isn't a shortcut; it's the most common reason new fish die within weeks of purchase.
How many fish can I keep in my tank?
It depends on the adult size of the fish, how messy they are, your filtration, and your maintenance routine. Skip the "1 inch per gallon" rule - it's inaccurate. Use a proper stocking calculator based on adult size and bioload, and always stock below your theoretical maximum to give yourself a buffer.
Why do my fish keep getting ich?
Ich (Ichthyophthirius multifiliis) is present at low levels in most tanks and only becomes a problem when fish are stressed enough for their immune systems to fail. The most common triggers are temperature drops (adding cold water during changes, heater failure), overstocking, and poor water quality from infrequent changes. Fix the stress cause first - treating ich while the stressor remains means it'll return.
How often should I do water changes?
Weekly for most tanks, at 25-30% per change. Lightly stocked, heavily planted tanks can sometimes go fortnightly, but this requires regular nitrate testing to confirm. Never skip water changes because the water looks clear - nitrate is invisible, and it accumulates regardless of appearance.
My fish are active and eating well but my water test shows ammonia - do I need to act?
Yes, immediately. Any detectable ammonia (above 0.25 ppm) in an established tank means something is wrong - an overstocked tank, a dead fish you haven't found, filter failure, or a bacterial colony that's been compromised. Do a 30-50% water change right away to dilute it, find and remove the source, and retest in 24 hours. Don't wait for fish to show symptoms.