🧮 Step 1 — Know What Your Filter Can Process

• Mature freshwater biofilters (3+ months old, healthy media) can usually oxidize about 0.2–0.3 mg TAN (total ammonia nitrogen) per liter per day.
• Example: 400 L tank × 0.25 mg/L/day = ~100 mg TAN/day your filter can handle.

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🐟 Step 2 — Convert Fish Biomass to Ammonia Load

• Average tropical fish excretion ≈ 30 mg TAN per 100 g fish per day at 25–28 °C.
• Large messy species (plecos, cichlids, stingrays) can be 2–3× that (≈60–90 mg TAN/100 g/day).

👉 That means:

• 100 g of small community fish → ~30 mg TAN/day.
• 100 g of large messy fish → ~60–90 mg TAN/day.

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📊 Step 3 — Apply Weekly Addition Rule

Safe increase = 10–20% of existing biomass/week (max)

• Mature tank with community fish → add ~100 g fish per 500 g existing stock (per week).
• Mature tank with plecos, cichlids, rays → add ~50 g per 500 g existing stock (per week).

Newer filters (<2 months old)

• Add ≤50 g of small fish every 2 weeks.
• Wait until ammonia + nitrite are consistently 0 before adding again.

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📏 Concrete Example

Scenario: 400 L tank, already holding 2.0 kg fish (mixed plecos + cichlids).

• Current load: ~2,000 g fish → ~1,200–1,500 mg TAN/day produced.
• Filter capacity: ~100 mg TAN/day per 100 L → ~400 mg/day. (Your biofilter clearly must already be scaled to the current load, but additions still risk spikes.)

Safe addition guideline:

• Add ≤200 g new fish/week (10% of biomass) if hardy species.
• If big plecos/rays: ≤100 g/week.
• Always pause and test before adding the next group.

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🧪 Step 4 — Use Tests as a Gatekeeper

After each addition:

• Test ammonia and nitrite daily for 5–7 days.
• If either stays >0.25 ppm → hold off, water change, wait for biofilter recovery.
• Only add more fish when readings are 0 consistently.

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✅ Quick Reference Targets

• Community tanks (guppies, tetras, barbs):
  +10–20% biomass/week (≈50–100 g fish in a 200–400 L tank).
• Messy fish (plecos, cichlids, goldfish):
  +5–10% biomass/week (≈50–100 g in a 400 L tank).
• Stingrays / monster setups:
  +≤5% biomass/week (≈50 g in a 400 L tank).
• New filters:
  1–2 small fish every 2 weeks until mature.

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⚡ Rule of thumb you can run with:

In a 400 L tank with an established filter, you can safely add about 50–200 g of fish biomass per week, depending on species messiness.