10 fishkeeping myths that need to die

Myth 1: Fish need to be fed every day

Overfeeding is one of the most consistent causes of poor water quality in home aquariums. Uneaten food sinks, decays, and produces ammonia and nitrate. Even eaten food produces ammonia through digestion. Feeding twice daily — a common recommendation — is appropriate for many species, but the universal anxiety about fish going hungry leads keepers to overfeed dramatically.

Most healthy tropical fish do well on one feeding per day. Many experienced hobbyists use one fasting day per week as a standard practice — it allows the digestive system to clear, reduces waste production, and doesn’t harm healthy fish in the slightest. In the wild, fish do not eat on a schedule. They eat when food is available, which is variable. A 24-hour fast for a healthy adult fish is a non-event biologically.

The guide rule: feed what your fish consume completely in 2 minutes, once or twice daily. Remove any uneaten food after 5 minutes. If fish aren’t eating, that’s information — not a prompt to feed more.

Myth 2: Tapping on the glass is harmless

Fish possess a lateral line system — a series of sensory organs running along their sides that detect pressure changes and vibrations in the water. This system is how fish sense the movement of other fish, detect approaching predators, and navigate in low visibility. It is exquisitely sensitive to pressure disturbances.

A sharp tap on the glass produces a pressure wave inside the tank that is directly transmitted to the lateral line. To a fish, this is equivalent to a sudden, unexpected concussive impact — not a gentle curiosity. Fish that are tapped at regularly show chronic stress responses: elevated cortisol, reduced immune function, increased susceptibility to disease, and behavioural changes including hiding and reduced feeding. The impulse to tap on the glass to “wake up” or “play with” fish is understandable and usually comes from a good place. But it is actively harmful when done repeatedly. Observe your fish — don’t knock at them.

Myth 3: A tank eventually reaches natural balance and doesn’t need water changes

This one circulates in natural and biologically-inspired tank communities and has a surface plausibility that makes it appealing. The idea: enough plants, the right balance of fish, and the tank becomes a self-sustaining ecosystem that needs no intervention. It’s a beautiful concept. It is not how closed aquarium systems work.

The problem is nitrate. In the nitrogen cycle, ammonia is converted to nitrite, then to nitrate. Nitrate is the end product of biological filtration in a standard tank — and it has no natural exit pathway in a closed system without plants consuming it fast enough to match input, which essentially never happens in a normally stocked tank. Nitrate accumulates continuously. Above 40 ppm, it causes chronic stress. Above 80–100 ppm, it starts causing direct health problems. Above 200 ppm, it becomes acutely toxic.

Regular water changes — typically 20–30% weekly — are the only reliable way to dilute and remove nitrate over time. They also replenish minerals that are consumed or precipitate out of the water. There is no substitute. The “natural balance” aquarium is a compelling concept; it is not a maintenance-free reality.

Myth 4: Fish grow to the size of their tank

This myth is so damaging that it gets its own dedicated guide — see our guide on the fish-grows-to-tank-size myth. In summary: what appears to be growth adaptation is stunting. A fish that has “stayed small” in a small tank has had its external growth suppressed by the build-up of growth-inhibiting hormones in inadequate water volume. Its internal organs continue developing. The result is organ compression, chronic health problems, and a dramatically shortened lifespan. Stunting is not adaptation. It is slow harm.

Myth 5: Enough plants means you don’t need water changes

A heavily planted tank with good lighting consumes significant nitrate. The “Walstad method” — a specific approach using nutrient-rich soil, minimal filtration, and dense planting — can genuinely reduce water change frequency in a lightly stocked, heavily planted tank. But this is a specific, carefully balanced setup. “I have some plants, so I don’t need water changes” is a completely different statement, and it’s wrong.

Plants consume nitrate in proportion to their growth rate. A tank with slow-growing plants, moderate lighting, and a reasonable fish load will still accumulate nitrate faster than the plants can process it. The only way to know is to test. If nitrate is staying below 20 ppm between weekly water changes without you doing those changes — the plants are doing the work. If nitrate is climbing, they’re not. Test your water; don’t assume.

Myth 6: Dead fish should be flushed down the toilet

This is wrong on two counts. First, the practical one: flushing a dead fish can introduce disease organisms or invasive parasites into natural waterways via the sewage system. In countries with combined sewage overflows (which is most countries), what goes down the toilet does not always stay contained. The risk is real but difficult to quantify.

Second, a dead fish rotting in a tank produces significant ammonia extremely quickly — but the solution is not flushing. Remove the fish promptly and dispose of it in the household waste (double-bagged), or bury it in your garden. If the fish died of disease, incineration or sealed bin disposal is preferable. Do not flush. The plumbing mythology about flushing fish was probably more romantically convenient than responsible.

Myth 7: Aquarium salt is good for all fish tanks

Aquarium salt — sodium chloride added to fresh water — is a legitimate and useful treatment in specific contexts: it helps fish produce slime coat in response to stress, provides electrolytes, and at therapeutic doses has some effect on certain parasites (ich) and bacterial infections. For livebearers and goldfish, a low background salt level is often recommended. None of this makes it appropriate for all tanks.

Soft water species — neon tetras, discus, corydoras, most Amazonian fish — are genuinely harmed by salt in their water. Shrimp and many invertebrates are highly salt-sensitive. Scaleless fish (some catfish) absorb salt through their skin more readily than scaled fish and can be damaged by concentrations that scaled fish tolerate. Salt in these tanks causes osmotic stress. Use salt when there’s a specific therapeutic reason, in the appropriate concentration, for the appropriate species. Not as a general additive to every tank.

Myth 8: Every tank needs an airstone

Airstones oxygenate water by creating surface agitation. In tanks with sufficient surface movement — from a filter return, a powerhead, or a spray bar — additional surface agitation via an airstone adds little. What matters is gas exchange at the water surface: oxygen entering, carbon dioxide leaving. Any filter setup that creates adequate surface agitation is doing this effectively.

An airstone is useful when: you have a low-flow setup, a heavily planted tank where CO₂ is deliberately supplemented (airstone at night helps purge excess CO₂), or a hospital tank where you want maximum oxygenation for stressed fish. It is not a universal requirement. A well-positioned filter return that creates gentle rippling at the surface is perfectly sufficient for most setups.

Myth 9: Medication cures disease

Medication manages disease. The distinction is not semantic. A fish that gets ich, is treated with medication, and appears to recover is not necessarily cured — ich has a life cycle that includes a free-swimming stage, a encysted stage (under the substrate), and the visible trophont stage on the fish. Medication kills the free-swimming stage. If treatment ends before the encysted stage completes its cycle and the fish’s immune system remains compromised, the disease can return. “The spots went away” is not the same as “the parasite life cycle has been fully broken.”

More broadly: the best fishkeeping outcome is prevention — appropriate tank size, cycling, stable parameters, quarantine of new fish — not treatment of predictable disease. A fish in chronic stress will get sick repeatedly regardless of medication available. Fix the conditions; don’t just treat the symptoms.

Myth 10: All algae is bad and should be eliminated

Algae is the bane of many fishkeepers’ lives, and managing it is a legitimate part of aquarium maintenance. But the framing of algae as universally bad — to be scrubbed off every surface, eliminated entirely — misses some important nuance:

• Biofilm and green algae on rocks and glass is food for algae-grazing invertebrates. Nerite snails, Otocinclus, and shrimp consume it continuously. A tank with zero algae provides less food for these animals.
• Green water (phytoplankton bloom) in a bare fry tank is actively beneficial — the fry consume it as a first food, improving growth and survival.
• Coralline algae equivalent in freshwater — some encrusting algae on rocks is aesthetically pleasant and biologically inert.

The algae types that are genuinely problematic are those that compete with plants for light and nutrients (hair algae, black beard algae) or those that indicate a specific water quality problem (blue-green algae, which is actually a cyanobacterium, indicates stagnation and low-flow areas). The goal should be balance, not elimination. A tank with some green growth on the back wall and decor is not a failing tank — it’s often a healthy one.