Essential knowledge for every fishkeeper - from beginner basics to advanced techniques.
The fish sold at two inches is a foot-long herbivore that can still be alive in twenty years, and the stocking decision has to be made for that animal rather than the one in the bag. Adults rasp surfaces, lip-lock with anything their own shape, and strip soft plants. Choose tank mates that will still be an appropriate size in five years.
Read Article
A betta will share a tank, but only with fish that stay out of the top third of it and carry nothing that looks like a rival's fins. Calm, quick, plain-coloured company works; anything slow and trailing, or anything that flares back, does not. The individual fish has the final say, so plan an exit for the tank mates before you buy them.
Read Article
Angelfish start as an easy community fish and finish as a tall, territorial pair that owns the middle of the tank. The safe company is fish too large to swallow, too fast to catch and not interested in trailing fins. Anything small enough to fit in an adult's mouth eventually will.
Read Article
Pea puffers are curious, slow-moving and far bolder than their size suggests, and the trouble they cause is nipping rather than fighting. Fast, small, plain fish that keep to the bottom or dart through open water fare best. In a small tank a puffer runs out of things to investigate and starts on its tank mates.
Read Article
Almost everything about discus company is decided by water rather than temperament. They need 28–30 °C and very low nitrate, and that narrow window rules out most of the standard community list before behaviour is even considered. What is left are warm-water, low-waste fish calm enough not to outcompete a slow, deliberate feeder.
Read Article
An electric blue acara is a cichlid that behaves like a community fish until breeding season, then claims a patch of the bottom and defends it. Mid-water and surface fish barely notice; other bottom dwellers do. Size is the second filter — anything small enough to be a mouthful eventually becomes one.
Read Article
An oscar eats anything it can swallow, produces waste on a scale most filters are not sized for, and rearranges the tank for entertainment. Company has to be big, fast, robust and cheap to feed, and it only works in a tank far larger than the oscar alone needs. Most oscars end up better off kept on their own.
Read Article
A common goldfish is a large, cool-water, heavy-waste animal, and those three facts eliminate almost every tropical fish sold as a tank mate. What remains are temperate species that tolerate the same 18–22 °C and are too big to be inhaled during feeding. Volume spent on tank mates is volume the goldfish was using to dilute its own waste.
Read Article
Fancy goldfish swim badly and see badly, so they lose every race to food and every chase they are drawn into. Tank mates have to be slow, cool-water and utterly uninterested in trailing fins and bulging eyes. Even well-chosen company usually means feeding the goldfish separately.
Read Article
With tiger barbs the host is the problem. In small groups they redirect their pecking order onto whatever else is in the tank; in a group of twelve or more they mostly keep it among themselves. Choose fast fish with short fins, and fix the group size before choosing anything at all.
Read Article
A dwarf gourami claims the surface and gets picked on everywhere else. It needs quiet, short-finned fish that live lower in the tank and no second surface-dweller to argue with. Buying from a source you trust matters too — iridovirus is common in the trade and no tank mate choice compensates for it.
Read Article
Guppies are usually the ones being hurt, not the ones causing harm: slow, bright, long-finned and breeding constantly. The list is shaped by three things — no fin nippers, nothing large enough to hunt them, and hard, alkaline water. Fry survival is the other half of the decision, since most tank mates treat them as food.
Read Article
Kuhli loaches spend the day hidden and the night working the substrate, so most conflicts are about food and floor space rather than aggression. They need soft sand, plenty of cover and calm mid-water fish that leave sinking food alone long enough for them to find it. Anything big enough to swallow a thin, eel-shaped fish is out.
Read Article
A rainbow shark patrols a stretch of the bottom and drives off anything that looks like another bottom dweller. Peaceful mid-water and surface fish are largely ignored; loaches, catfish and its own kind are not. Length of tank and number of separate hiding spots decide how this goes far more than the species list does.
Read Article
A red tail shark is the same territorial idea as a rainbow shark, only shorter-tempered and less willing to share a cave. It works with fast, open-water fish that never settle on the substrate, and fails with anything that competes for the floor. One per tank, always.
Read Article
A peacock eel buries itself in sand by day and hunts the bottom at night, which makes substrate and feeding the deciding factors. Company should be mid-water, calm and too large to be considered dinner — small shrimp and tiny fish will disappear. It also escapes, so the lid matters as much as the tank mates.
Read Article
An African dwarf frog is nearly blind and finds food by smell, so it starves in tanks full of quick eaters long before anything bites it. Suitable company is slow, small-mouthed and not competing for sinking food, and the tank has to be shallow enough for an easy trip to the surface. Anything small enough to fit in the frog's mouth is food.
Read Article
We compared the rule against the real minimum tank size of every species we track. It under-predicts by 2.26x per fish and 7.72x per group — and the reason is schooling, not size.
Read Article
What wattage a 20 gallon really needs, how a cold room changes the answer, and the buying criteria that separate a good heater from a cheap one.
Read Article
Why nano tanks need more watts per gallon, which heater format actually fits a 5 gallon, and how to avoid cooking a betta.
Read Article
The 50 W standard, when to step up to 100 W for a hospital tank, and what to check before you buy a heater for a small aquarium.
Read Article
Why 75 gallons is the point to split into two heaters, how to place them along a four-foot tank, and what build quality is worth here.
Read Article
One 300 W heater or two 150 W units? The honest trade-off on a four-foot 55, plus wattage for warm and cold rooms.
Read Article
Identify brown, green spot, hair, staghorn, black beard and blue-green algae from what you can see, then fix the actual trigger behind each one.
Read Article
Kuhli, clown, yoyo, hillstream, weather, dwarf chain and rosy loaches compared — real tank sizes, temperature, pH and group minimums from our database.
Read Article
Every cichlid type worth knowing, grouped into New World, dwarf, African rift lake and centerpiece species — with real tank sizes, temperament and pH.
Read Article
Every popular tetra species compared side by side — neon, cardinal, ember, rummy-nose, congo and more, with honest picks for community, blackwater and dither roles.
Read Article
The 15 best nano fish for 20–60 L tanks — chili rasboras, CPDs, ember tetras, scarlet badis and more, with real tank sizes and behaviour notes.
Read Article
Which algae eater actually works for your algae type and tank size — otos, bristlenose, amanos, nerites and the one common species we tell you to skip.
Read Article
Nine pleco species compared — from the 10 cm clown to the 45 cm common — with real adult sizes, tank minimums, temp and pH from our database.
Read Article
The largest home aquariums — fire eels, knifefish, frontosa colonies and piranha shoals with room to spare.
Read Article
A statement monster tank — fahaka puffers, big cichlid colonies and deep piranha shoals.
Read Article
Monster-keeping territory — frontosa colonies, piranha shoals and true predator displays.
Read Article
34 species that reward you for going big — where monster fish become manageable and dream setups become reality.
Read Article
A room-sized centerpiece — full-grown oscars, flowerhorns and archerfish with real estate to spare.
Read Article
32 species that flourish at this size — from discus groups to impressive cichlid communities.
Read Article
A true large-tank threshold — oscars, discus groups and big communities finally become responsible choices.
Read Article
30 species that make the most of this versatile tank size — from showstopper cichlids to massive schools.
Read Article
27 species that genuinely thrive in this popular tank size — from schooling tetras to stunning centerpiece cichlids.
Read Article
The tall alternative to a 55 — peacock cichlid displays and shark-and-barb communities that use the height.
Read Article
Where an aquarium becomes furniture — mbuna setups, medium cichlids and full communities.
Read Article
The sweet spot between starter and serious display — angelfish pairs, rainbow schools and dwarf cichlids.
Read Article
34 species that unlock at 40 gallons — Angelfish, Rainbowfish, and active swimmers that finally have room to thrive.
Read Article
37 species that genuinely thrive in a 30 gallon — from schooling tetras to centerpiece cichlids and active barbs. Honest picks based on adult size, bioload, and temperament.
Read Article
22 species that genuinely thrive in a 20 gallon — from schooling fish to centerpiece options. Honest picks based on adult size, bioload, and temperament.
Read Article
7 species that genuinely thrive in 5 gallons — not just survive. The honest guide to nano tank stocking.
Read Article
12 species that genuinely thrive — not just survive. Honest advice on what actually works in small aquariums.
Read ArticleMore guides coming soon! Have a topic you'd like us to cover? Let us know.
