My betta is in a 20-gallon planted tank with cardinal tetras and they seem fine. How long will this last?

Most betta-cardinal conflicts surface within 2-3 months as the betta matures. The apparent peace in the first few weeks is not a reliable indicator of long-term compatibility.

Will my betta eat cardinal tetras?

Possibly. Cardinals are 2 inches and fit in a betta's mouth. Unlike faster tetras, cardinals are slower swimmers and more likely to be caught, especially at night or during feeding frenzies.

Do bettas or cardinal tetras need softer water?

Cardinal tetras need softer, more acidic water (pH 5.0-7.0) than bettas (pH 6.5-7.5). Bettas tolerate a wider range but do not thrive at the lower end of what cardinals prefer.

What is a safe tankmate for both a betta and cardinal tetras?

There is no shared species that both bettas and cardinal tetras thrive with. Keep bettas with peaceful bottom-dwellers; keep cardinals in schools with other soft-water tetras and rasboras.

Can Betta Fish Live With Cardinal Tetra?

By Alex WalshLast reviewed Apr 18, 2026

No, cardinal tetras are small schooling fish that need soft acidic water, conditions that conflict with standard betta setups.

Why

Cardinal tetras stay at 2 inches and fit easily in a betta's mouth if the betta decides to hunt them, especially at night when tetras rest near the substrate.
Cardinal tetras need soft, acidic water (pH 5.0-7.0) while bettas prefer slightly higher pH (6.5-7.5). Finding a sweet spot that benefits both is difficult without compromising one species.
Cardinal tetras are schooling fish that need groups of 8+ in a 20+ gallon tank, which conflicts with the smaller, single-species betta setups most people run.
A betta in a planted 20-gallon might tolerate cardinal tetras initially, but aggression increases as the betta matures and claims its territory, typically within 2-3 months.

Minimum shared tank

15 gallons

Shared temperature range

76-81°F

What could go wrong

Cardinal tetras will either be chased to exhaustion or eaten one at a time over weeks. The red coloration makes them highly visible targets for a territorial betta, and losses often go unnoticed until the school is severely depleted.

If you keep them in separate tanks

Sized for the 15 gallon minimum this pairing needs. Affiliate links — we earn a small commission if you buy through them.

AquaClear 30 Power Filter

Oversized filtration is what lets you stock two species together without water quality crashing. Rated for a tank slightly larger than the 15 gallon minimum.

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Eheim Jager 150W Thermostat Heater

Holds a steady temperature inside the 76-81°F window both species need.

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API Freshwater Master Test Kit

Keep ammonia, nitrite, and nitrate in check. Two-species tanks have more bioload and less margin for error than single-species setups.

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Better pairings to consider

Bettas do well with corydoras and snails; cardinal tetras pair with neon tetras, rummy nose tetras, harlequin rasboras, and corydoras in a soft-water community tank.