Freshwater Fish Species Reference Chart

Sources disagree on the exact floor — some hobbyist forums cite 10 gallons as a practical minimum for a school of 6, while other care guides recommend 15-20 for better long-term water stability; this table uses the more commonly cited 10-gallon figure. Must be kept in a group of 6 or more: smaller schools show clamped fins, faded color, hiding, and shortened lifespan from perceived predation risk. Some current guides recommend 8+ for a fuller, more secure-feeling school — 6 remains a widely-cited practical floor, but more tank length matters more here than the extra 2 fish.

Males run smaller (~1.2in) and females noticeably larger (~1.8-2.4in) — this table uses a representative figure closer to the female size most commonly kept in numbers. Livebearers that breed readily; a small starting group can quietly outgrow a tank's bioload capacity within months if fry aren't managed. Keep at least 1 male to 2-3 females, not a 1:1 ratio — a lone female receiving constant mating attention from a single male shows chronic stress and can die prematurely from the harassment alone, independent of tank size.

Another livebearer — expect ongoing fry from a mixed-sex group, which adds to bioload over time even though this table's number is per adult. Same welfare guidance as guppies: keep at least 1 male to 2+ females rather than a 1:1 pair, to avoid one female being over-harassed.

"Molly" covers several sizes — short-fin common mollies run smaller, sailfin mollies noticeably larger (up to 5-6in) and need more room than this table's representative figure. Also a livebearer; a mixed-sex group breeds continuously — use the same 1 male to 2+ female ratio guidance as guppies and platies to avoid over-harassing a single female.

Sources range from a bare 2.5 gallons up to 10+ gallons as "minimum"; this table uses 5 gallons, commonly cited as the floor for stable temperature and water chemistry, matching current consensus. Territorial, especially toward other bettas (never house two males together) and toward long-finned or brightly colored tankmates that can trigger flaring. Keeping more than one betta isn't primarily a volume question this table's per-fish gallon increment can capture — males will often fight regardless of tank size, and even a divided or heavily planted multi-betta setup needs real experience and close monitoring, not just more gallons.

A fast, active swimmer that needs horizontal swimming room, which is why the minimum tank size (a 20-gallon long, not just 20 gallons of any shape) is driven more by footprint than by bioload. Keep in a school of 6 or more. Generally a good community fish, but a documented occasional fin-nipper toward slow-moving, long-finned tankmates (e.g. bettas, fancy guppies, angelfish) — watch new introductions.

Covers medium corydoras species specifically (panda, trilineatus, julii, ~2-2.2in). Dwarf species (pygmy, habrosus, hastatus, ~1-1.2in) are smaller and not separately listed here — don't apply this row's numbers to them. Peaceful bottom-dwelling schooler; keep in a group of 6 or more.

6in is body length — fin span can add well over a foot of vertical height, so a tall tank matters as much as gallons. 29 gallons is commonly cited as a floor for a single adult or pair; a school of 4+ angelfish is more often recommended at 55+ gallons (the per-fish increment used here approximates that jump). A cichlid at heart: can turn territorial, especially when breeding, and may prey on fish or shrimp small enough to fit in its mouth (e.g. neon tetras, cherry shrimp).

Can survive at 10 gallons but sources consistently recommend 20+ for better quality of life, plus roughly 5 more gallons per additional fish. Can become territorial toward its own species or similarly shaped/colored fish in a tank on the smaller side. Buyer-beware note not about tank size: dwarf gourami iridovirus (DGIV), a real, peer-reviewed, incurable viral disease, has been documented in a meaningful share of farmed/imported stock (~22% in one Singapore-farm study) with near-total mortality once symptoms appear — quarantine new fish and buy from a source you trust, since no tank size fixes this.

30 gallons is cited for a full school of 8, which this table treats as the real minimum group size — sources are specific that keeping fewer redirects this species' natural aggression onto tankmates rather than within the school itself. A well-known fin-nipper even at a full school size — incompatible with slow-moving, long-finned fish (e.g. bettas, fancy guppies) and with shrimp.

Males can reach 5in. 20 gallons is a commonly cited single-fish floor; a male/female pair is more often recommended at 30+ gallons. Peaceful toward other species but can be territorial toward its own kind over hiding spots. Meaningfully messier than a similarly sized community fish, but much less so than a common pleco — don't confuse the two when buying (see the common pleco entry).

Frequently sold small (~2in) in a generic "algae eater" tank, but reaches 18-24in+ as an adult. Sources are genuinely split on the real minimum tank size (55/75/100+ gallons all cited) — this table uses the more conservative 100-gallon figure given how large and messy this fish gets; a fish this size reliably outgrows a 55-gallon tank within a few years, and even 75 gallons is contested as too tight by several sources. Check adult size before buying, not just what's in the store tank. A heavy, messy bottom feeder whose waste output some sources compare to roughly ten large tetras; adults can also rasp the slime coat of slower, flat-bodied tankmates (e.g. angelfish, discus, goldfish) and can become more territorial with age.

Reaches 10-14in typically, some individuals larger. 75 gallons is the current consensus minimum for a single adult; an older, still commonly repeated figure of 55 gallons is now explicitly called outdated by more recent care guides, and some sources recommend 100+. A territorial cichlid that will eat fish and invertebrates small enough to fit in its mouth — not a community-tank fish. A single oscar produces disproportionately more waste than its length alone suggests (large carnivorous cichlids are messy, heavy eaters), which this table's bioload factor reflects with a substantially higher-than-average per-inch weighting.

Single-tail (common/comet/shubunkin) goldfish reach 12-18in in a large enough tank or pond — this is not a bowl fish. Sources disagree on the exact minimum (30-40 gallons cited for the first fish); this table uses the more conservative 40 gallons, plus roughly 10-15 more per additional fish. Goldfish lack a true stomach and feed/excrete almost continuously, needing strong filtration regardless of the bioload percentage below. Two compatibility issues not captured by the temperament label: goldfish are cold-water/temperate fish (comfortable around 65-75F) rather than tropical (most fish in this table want 75-82F), making most of this table poor tankmates on temperature grounds alone; and an adult goldfish is large enough to eat small fish and shrimp it can fit in its mouth.

A single shrimp can live in as little as 5 gallons; a breeding colony is more often recommended at 20+ gallons. A documented healthy density for Neocaridina shrimp (the common "cherry shrimp" species) is around 10 shrimp per gallon in a mature, planted tank — this table's per-shrimp gallon increment is derived directly from that figure. Produces minimal bioload for its size, but is itself vulnerable prey to many semi-aggressive or larger fish (tiger barbs, angelfish, goldfish, and others in this table) — check predation risk separately from the bioload math below.