34 fish, shrimp, snails, amphibians, and reptiles, with the parameters and conflicts that decide what shares a system. Match the animal to the water and the space you can hold.
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Betta splendens
One male per tank; two males fight to the death. Needs a heater at 78 to 82 F and calm flow. Heavy plant cover lets some keepers add shrimp, but a hungry betta still hunts them.
Poecilia reticulata
A hard-water livebearer: they want pH 7.0 to 8.0 and GH 8+, the opposite of soft-water tetras. Buy all-male or all-female unless you want an endless supply of fry.
Poecilia wingei
A nano guppy: same hard-water needs, smaller and less finicky. Peaceful enough to keep with dwarf shrimp, and small enough for a 5-gallon.
Paracheirodon innesi
A soft-water schooler: keep six or more or they hide and fade. Wants pH under 7.0 and soft water, which puts it at odds with guppies and mollies.
Hyphessobrycon amandae
A tiny orange schooler that glows against green plants. Peaceful with dwarf shrimp adults, though it may pick off newborn shrimp. Keep eight or more.
Boraras brigittae
One of the few true nano fish that suits a 5-gallon: deep red, under an inch, and safe with adult shrimp. Wants soft, acidic, warm water and a tight lid.
Trigonostigma heteromorpha
A hardy, forgiving schooler with a black wedge marking, more tolerant of parameter swings than a neon. A good first schooling fish for a planted 10 to 20 gallon.
Tanichthys albonubes
The best unheated-tank fish: it prefers 60 to 72 F and does poorly in warm tropical water. That cool preference makes it ideal for a filterless Walstad tank in a normal room, and it summers happily in a pond.
Corydoras aeneus
A social bottom fish that works the substrate for leftover food; keep six or more or they get stressed. Wants smooth sand, not sharp gravel, which wears their barbels.
Corydoras pygmaeus
A one-inch cory that schools in mid-water as much as on the bottom. The right bottom fish for a nano, and safe with adult shrimp.
Otocinclus sp.
A true algae-eater that stays under two inches and never harms plants. Only add it to a mature tank (three-plus months) with a steady biofilm supply, or it starves.
Trichopsis pumila
A tiny labyrinth fish that croaks audibly and stays under two inches. A nano-tank centerpiece that suits the warm, calm water a betta likes but is far more peaceful.
Trichogaster chuna
The peaceful gourami: a two-inch, honey-colored centerpiece that suits a planted 10 to 20 gallon and, unlike its cousins, rarely bullies tankmates.
Danio rerio
A tough, active schooler that tolerates cooler water (64 to 77 F) and forgives beginner mistakes. So active it stresses slow or long-finned tankmates, so pair it with other quick fish.
Danio margaritatus
A jewel-spotted nano fish, once called the galaxy rasbora. Shy at first: heavy planting and a group of six or more brings them into the open.
Xiphophorus maculatus
A hardy hard-water livebearer, even more forgiving than a guppy. Wants alkaline water (pH 7.0 to 8.2, GH 10+) and breeds freely, so plan for fry or keep one sex.
Carassius auratus
A cool-water fish that eats soft plants and produces a heavy bioload, so it belongs in a pond or a large tank, not a jar. It uproots and grazes anything but the toughest plants (Anubias, Java fern, Vallisneria).
Neocaridina davidi
The beginner shrimp and the best cleanup animal in a nano tank: it grazes biofilm and leftover food all day. Needs minerals in the water (GH 4 to 14) for molting, and copper (in some fish meds and fertilizers) kills it. A mature, planted tank is safest.
Caridina multidentata
The best algae-eating shrimp, twice the size of a cherry and too big for most fish to eat. It will not breed in freshwater (larvae need brackish), so the population stays stable.
Caridina cantonensis
The advanced shrimp: it needs soft, acidic water (pH 5.5 to 6.8) over an active buffering substrate and stable parameters. Do not mix it with cherry shrimp (they crossbreed) or keep it in hard water.
Palaemonetes paludosus
A cheap, transparent scavenger often sold as feeder shrimp. Hardier and more tolerant of hard water than a cherry, though larger ones can occasionally nip tiny fish.
Neritina sp.
The best algae-eating snail that will not overrun the tank: it lays eggs but they never hatch in freshwater. Wants harder, alkaline water (GH 6+) or its shell pits and erodes.
Pomacea bridgesii
A large, colorful snail that scavenges without eating healthy plants (it takes only softening leaves). Needs calcium-rich, harder water for its shell, and it lays pink egg clutches above the waterline you can simply remove.
Planorbella sp.
A flat-coiled cleanup snail that breeds to match the available food, so a population explosion means you are overfeeding. Harmless to healthy plants and a good part of a self-sustaining cleanup crew.
Melanoides tuberculata
A burrowing snail that aerates the substrate and eats detritus below the surface, which makes it genuinely useful in a dirted Walstad tank. Livebearing and nocturnal; the population self-limits to the food supply.
Physella acuta
The snail that arrives free on new plants. It gets a bad name, but it is a fast, harmless cleaner that only booms when there is excess food. Tolerates the widest range of any common snail.
Dendrobates auratus
The beginner dart frog: bold, day-active, and hardy within a narrow band (72 to 80 F, humidity 80 percent-plus). It eats the springtails and fruit flies a bioactive viv produces. Captive-bred frogs are not toxic, but never house them over 82 F.
Dendrobates tinctorius
The largest common dart frog and one of the boldest, in dozens of color morphs. Keep one, a proven pair, or a group with space; two females often fight. Same care band as auratus.
Litoria caerulea
A hardy, chunky tree frog that tolerates handling better than most and does not need the extreme humidity a dart frog does. It eats anything it can fit in its mouth, so no small tankmates.
Correlophus ciliatus
The best beginner arboreal reptile: no heat lamp needed (room temperature 72 to 78 F is ideal), and a commercial powdered diet covers most of its food. Keep one per enclosure; two males will fight. Never let it exceed 80 F.
Rhacodactylus auriculatus
A close relative of the crested gecko with the same room-temperature, no-lamp care, and a slightly wider humidity tolerance. Housed one per enclosure. A sturdy bioactive-vivarium reptile.
Lepidodactylus lugubris
The only common gecko you keep in a group: it is parthenogenetic (all female, reproducing without males), so a colony grows on its own. Small enough for a planted 12x12x18, and it eats the same micro-insects a dart viv produces.
Pangio kuhlii
An eel-like, peaceful bottom fish that hides by day and forages at night; keep five or more or it never comes out. Wants soft, warm water and fine sand it can burrow into, plus caves and leaf litter for cover.
Geosesarma dennerle
A tiny, purple-and-orange land crab for a planted paludarium: it needs mostly land with a shallow water dish, not a full aquarium. Groups peacefully with plenty of hides, and it eats the same detritus the cleanup crew works.
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