Sample beginner tank: 20 gallons

Built November 9, 2026 from the TankScore Species Ledger (data checked November 9, 2026).

1. Before and after

Now
0/100 · bioload 184%
11 issues

→

After this plan
88/100 · bioload 115%
2 issues

What’s wrong now

critical Betta (male) (24–30°C) and Fancy goldfish (10–21°C) have no temperature in common.
major Tiger barb is a known fin nipper; Betta (male) has long fins.
major Tiger barb is a known fin nipper; Fancy goldfish has long fins.
major Heavily overstocked: about 184% of what this tank and filter can handle.
minor A bit small for Tiger barb: the cautious minimum is 30 gallons.
minor Tiger barb should be kept in groups of 8+ (you have 4).
minor Bronze corydoras should be kept in groups of 6+ (you have 3).
minor Tiger barb can bully much smaller, peaceful Neon tetra.
minor Betta (male) can bully much smaller, peaceful Neon tetra.
minor Neon tetra and Fancy goldfish share only a narrow temperature band (21°C).
minor Bronze corydoras and Fancy goldfish share only a narrow temperature band (21°C).

2. The changes

Rehome or don’t buy the 1 × Fancy goldfish. Betta (male) (24–30°C) and Fancy goldfish (10–21°C) have no temperature in common.
Your tank is full after these changes, so you don’t need a replacement. With a bigger tank or an oversized filter, these fit with the rest: 1 × Nerite snail · 1 × Ramshorn snail · 1 × Ghost shrimp · 1 × Amano shrimp · 6 × Otocinclus
Rehome or don’t buy the 4 × Tiger barb. Tiger barb is a known fin nipper; Betta (male) has long fins.
Your tank is full after these changes, so you don’t need a replacement. With a bigger tank or an oversized filter, these fit with the rest: 8 × Black phantom tetra · 6 × Glowlight tetra · 8 × Lemon tetra · 8 × Harlequin rasbora · 6 × Head-and-tail-light tetra
Bronze corydoras: 3 → 6. Bronze corydoras needs a group of 6+ to feel secure.
Neon tetra: 10 → 8. Brings the bioload down to what this tank and filter can handle.
Tank or filter. Even at minimum group sizes this stocking is about 115% of capacity. A bigger tank or an oversized filter is the fix, or remove one species.

Still worth knowing: Betta (male) can bully much smaller, peaceful Neon tetra. Slightly overstocked: about 115% of capacity. More filtration or fewer fish helps.

Final stocking: 1 × Betta (male) · 8 × Neon tetra · 6 × Bronze corydoras

3. Water targets

Temperature 24–25 °C (75–77 °F). Set the heater in the middle of that range. pH 6–7. A stable pH matters more than hitting an exact number; don’t chase it with chemicals.

4. Stocking order

After the tank has cycled: 8 × Neon tetra
About 2 weeks later: 6 × Bronze corydoras
About 4 weeks later: 1 × Betta (male)

Add each batch only when ammonia and nitrite have read 0 for a week since the last one. If your fish are already in the tank, skip this and use the log to watch the changes.

5. Cycling checklist (fishless cycle)

☐ Set up the tank with the filter and heater running, and add water conditioner.
☐ Add an ammonia source (pure bottled ammonia or a pinch of fish food) and, if you have it, media or bottled bacteria from an established tank.
☐ Test every 2–3 days with a liquid test kit: ammonia, nitrite, nitrate. Write the readings in the log.
☐ Wait for nitrite to appear, then fall. This usually takes 3–6 weeks.
☐ Cycled when the tank turns 2 ppm ammonia into 0 ammonia and 0 nitrite within 24 hours, and nitrate is rising.
☐ Do a large water change to bring nitrate down, match the temperature, then add your first batch.
☐ Weekly after that: change 20–30% of the water, test, and vacuum the gravel lightly.

Fish looking sick? This plan is not veterinary advice. Ask a local fish store or an aquatic vet.

6. 30-day tank log

Day	Date	Temp	Ammonia	Nitrite	Nitrate	Water change	Notes
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7. Your report card

Your card after the changes, with your tank’s name.

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