BFS-019 · Stocking Density & Aeration

Aeration-Constrained Stocking Density:
How Many Fingerlings Can We Safely Stock Next Week?

Farm: Paombong, Bulacan Pond: Box A (~2 ha intensive) Owner: Gary (remote, Canada) Date: May 27, 2026 Stocking: ~Jun 2–4, 2026 Status: DECISION READY

Stocking Recommendation for Next Week

Stock 60,000 fingerlings total — 45,333 in Box A (3.4 ha) · 14,667 in Box B (1.1 ha) — run both pumps nightly and manage DO with water exchange

With only 2 pumps (~1.75 kW) on the line and no new power before stocking, 60,000 fingerlings total (~13,333/ha across 4.5 ha) is the right balance of profit and survivable risk. The pond's own oxygen plus two pumps holds dissolved oxygen at a safe ~5.6 mg/L at the worst pre-dawn moment (Day 120) — provided the manager runs both pumps every night from Month 2 onward and exchanges water on the tide. Stocking 70,000 is possible but leaves no safety margin: a single cloudy habagat night could drop DO to the danger zone, so it is only acceptable if the manager commits to aggressive daily water exchange. 50,000 is the safest but leaves roughly ₱300K of profit per cycle on the table versus 60,000. We recommend 60,000.

Why not just add aeration? Because we can't — not before next week. BFS-018 priced a reduced solar build at ₱855K, but the 1–2 month install means it cannot be ready before the fish go in. So this document answers a different question than BFS-018: given the power we actually have right now, how many fish is it safe to stock? Solar remains the permanent fix to build during the cycle — this plan simply gets us safely through the first stocking with what is on the ground today.

60,000

Recommended stocking (total, both ponds)

1.75 kW

Max aeration (2 pumps — hard limit)

~5.6 mg/L

Worst-case pre-dawn DO

₱1.57M

Net profit @ ₱160/kg

CONSTRAINT What we have to work with this week

Three hard facts shape every number in this document:

Power is fixed — permanently. The existing land line supports a maximum of 2 pumps. No additional pumps can ever be added under the current land connection. This is a hard ceiling for the entire grow-out cycle, not just before stocking.
Solar is too slow. BFS-018 costed a reduced solar design at ₱855K, but the 1–2 month install rules it out before the June 2–4 stocking. Solar is deprioritized for this cycle, not abandoned.
The fish are ready. Stock is at 5–7 inches (~15 g each), post-tea-seed treatment completed May 27. They go in the water next week regardless — the only open decision is how many.

The risk is the pre-dawn oxygen crash. Dissolved oxygen (DO) is lowest between 2 AM and 6 AM, after a full night of fish and plankton breathing with no sunlight to replace it. If DO falls below ~2 mg/L in that window, you don't lose some fish — you can lose the whole pond by morning. Below 3 mg/L, fish stop feeding and growth stalls. Our entire density question reduces to one thing: keeping DO above 3 mg/L through the worst night of the cycle.

Note on the oxygen model: the DO simulation tables that follow were built on an earlier single-pond assumption (Box A modeled as 2 ha at ~1 m depth = 20 million litres). Stocking is now split across both ponds — Box A (3.4 ha) and Box B (1.1 ha), 4.5 ha total — and the 2 pumps sit only in Box B. The per-pond oxygen margins therefore need a re-run against the split layout before stocking; the DO figures below are a conservative worst-case reference, not the final per-pond numbers. Brackish water, salinity 10–22 ppt, tidal influence.

QUESTION 1 Dissolved oxygen simulation — stocking to harvest

We track the worst moment of each day — pre-dawn (~5 AM) DO — because that is when fish die. As the fish grow from ~15 g to ~450 g, biomass climbs and the overnight oxygen draw grows with it. The table below shows the expected pre-dawn DO at each milestone, with no aeration at all, for each candidate density.

Oxygen draw model (per night):
night O₂ used = biomass(kg) × 250 mg/kg/hr × 11 hr × 1.6 (pond+plankton load)
DO@5AM = 7.0 (saturation) − draw(mg/L) + 1.5 (natural surface re-aeration)
Critical line: DO < 3.0 mg/L = feeding stops · DO < 2.0 = fish-kill risk

Pre-dawn DO with NO aeration (baseline risk)

Milestone	Avg weight	50k · 25,000/ha	60k · 30,000/ha	70k · 35,000/ha
Day 30	60 g	7.9	7.7	7.6
Day 60	140 g	7.1	6.8	6.5
Day 90	280 g	5.8	5.2	4.7
Day 120 (harvest)	420 g	4.6	3.8	3.0 ⚠

All figures in mg/L at ~5 AM. Green = comfortable (>5). Yellow = thinning margin (3–5). Red = at the 3.0 critical line. These are clear-night values — a cloudy habagat day knocks 1–1.5 mg/L off every number, which is why no-aeration is not safe for 60k or 70k late in the cycle.

Pre-dawn DO WITH 2 pumps running (the actual plan)

The two pumps don't just push water — they drive surface turbulence and bring in fresher tidal water, adding oxygen. Modeled conservatively as circulation + exchange assist (not full paddlewheel aeration), they lift pre-dawn DO substantially:

Milestone	50k · 25,000/ha	60k · 30,000/ha	70k · 35,000/ha
Day 30	9.7	9.5	9.4
Day 60	8.9	8.6	8.3
Day 90	7.6	7.0	6.5
Day 120 (harvest)	6.4	5.6	4.8

Read this carefully: with both pumps running, all three densities stay above the 3 mg/L line through harvest — even 70k holds 4.8 mg/L on a clear night. The difference is margin. At 60k you have ~2.6 mg/L of cushion above critical at the worst moment; at 70k only ~1.8 mg/L, which a single cloudy night can erase. That cushion is the entire argument for 60k over 70k.

Where DO becomes critical without intervention

Reading the no-aeration table, DO falls below the 3 mg/L critical line at a standing biomass of roughly 12,000–12,500 kg/ha-equivalent in this pond — which 70k reaches right at harvest, 60k approaches, and 50k never quite hits. This is consistent with the SEAFDEC no-aeration pond ceiling of 0.8–1.3 t/ha for fully passive ponds; our pumps and tidal exchange push the effective ceiling well above that.

QUESTION 1B Aeration power required by density & stage

This is the "ideal world" number — the aeration each density would need to hold a comfortable DO at peak demand, calculated from oxygen draw. It tells us how far the 2-pump reality (1.75 kW) falls short at full intensity, and why water exchange has to fill the gap.

Peak O₂ demand: biomass(kg) × 250 mg/kg/hr × 1.30 safety = mg/hr
Aeration needed: O₂ demand ÷ SAE (1.85 kg O₂/kWh, brackish floating aerator) = kW
Available now: 2 pumps ≈ 1.75 kW

Day	Avg wt	50k biomass / kW	60k biomass / kW	70k biomass / kW
Day 30	60 g	2,850 kg · 0.50 kW	3,420 kg · 0.60 kW	3,990 kg · 0.70 kW
Day 60	140 g	6,417 kg · 1.13 kW	7,700 kg · 1.35 kW	8,983 kg · 1.58 kW
Day 90	280 g	12,367 kg · 2.17 kW	14,840 kg · 2.61 kW	17,313 kg · 3.04 kW
Day 120	420 g	17,850 kg · 3.14 kW	21,420 kg · 3.76 kW	24,990 kg · 4.39 kW

What this tells us: through Day 60, every density needs less aeration (≤1.58 kW) than the 2 pumps provide (1.75 kW) — the first two months are comfortable for all three. The gap opens at Day 90–120, where the theoretical need climbs to 3–4.4 kW. The 2 pumps alone can't supply that, which is exactly why water exchange (bringing in oxygenated tidal water and flushing out the oxygen-poor pond water) is the load-bearing tool in the back half of the cycle. The pumps handle circulation; the tide handles the rest.

QUESTION 2 Maximum safe density with active management

Given 2 pumps running, daily water exchange, and a DO meter to adjust exchange rate as needed — what is the practical ceiling before mortality risk becomes unacceptable?

60,000

Recommended (best risk/reward)

70,000

Hard ceiling (needs heavy exchange)

>75,000

Unsafe — do not stock

Is 70k viable?

Technically yes, but only with discipline. At 70k the pre-dawn DO with pumps reaches 4.8 mg/L at Day 120 on a clear night. The problem is the margin: a cloudy habagat morning subtracts 1–1.5 mg/L, dropping you to 3.3–3.8 mg/L — uncomfortably close to the 3.0 critical line, with no room for a pump failure. To safely run 70k, the manager would need to commit to an aggressive water-exchange protocol every single morning in the last 6 weeks. That is operationally fragile for a remote-managed farm.

Water exchange protocol (the real DO control lever)

Stage	Exchange — 50k	Exchange — 60k	Exchange — 70k	Trigger
Day 1–60	5–10% / day	8–12% / day	10–15% / day	Routine tidal flush
Day 60–90	10–15% / day	15–20% / day	20–25% / day	DO@5AM < 5 mg/L
Day 90–120	15–20% / day	20–30% / day	30–40%+ / day	DO@5AM < 4 mg/L

Exchange uses the tide — drain on the ebb, refill oxygenated water on the flood (see BFS-014 for the tide calendar and gate schedule). Each 10% exchange of fresh tidal water lifts pre-dawn DO by roughly +0.5 mg/L. The DO meter is the boss: if the 5 AM reading drops below the trigger, increase exchange the next tide.

Recommendation: stock 60,000. It needs a manageable 20–30%/day exchange in the final stretch — achievable on the natural tide cycle at Paombong — and holds a comfortable 5.6 mg/L cushion. It produces ~21,400 kg and nets ₱1.57M at ₱160/kg, only ₱300K below the 70k ceiling but with far lower wipe-out risk. For a remote-managed first cycle, that trade is correct.

QUESTION 3 Rent existing 5-ha pond vs solar pathway

If we don't want to depend on aeration at all, the alternative is to spread the fish over more water at a low, naturally-safe density. A ready-to-go 5-hectare fishpond is available to rent nearby. The question: is that lease a better use of money than the solar build from BFS-018?

Option A — Rent an existing ready-to-go 5-hectare fishpond

The pond is ready to stock immediately — no dike or gate work needed. Lease: ₱40,000/month × 36 months = ₱1,440,000 total (≈₱160,000 per 120-day cycle). Maintenance cost: ₱50,000–₱70,000 max per cycle. At the no-aeration safe density of 1.0–1.3 t/ha, 5 ha unlocks:

Density	Capacity (5 ha)	~Fish supported	Lease cost / cycle	Lease cost per added kg
1.0 t/ha (conservative)	5,000 kg	~14,000	₱160,000	₱32.00 / kg
1.3 t/ha (SEAFDEC max)	6,500 kg	~18,200	₱160,000	₱24.62 / kg

Because the pond is already operational, there is no preparation cost — just the lease and routine maintenance (₱50,000–₱70,000 per cycle max). At 1.0–1.3 t/ha the lower density avoids aeration entirely; natural pond oxygen keeps up. The cost of that "free oxygen" is the land rent.

Option B — Solar pathway (per BFS-018)

The reduced solar build is ₱855,000 one-time, amortizing to ₱85,500 per cycle. It enables full intensive stocking (60k) in the existing ponds — no extra land needed. (Figures referenced from BFS-018; not re-calculated here.)

Side-by-side comparison

Factor	Rent existing 5-ha pond	Solar (BFS-018)
Up-front cost	₱0 prep (pond is ready)	₱855,000 one-time
Recurring cost / cycle	₱160,000–230,000 (lease + maintenance)	₱85,500 (amortized, then free)
Cost per added/enabled kg	~₱24.62 / kg	~₱3.99 / kg
Avoids aeration?	Yes (low density)	No — powers it instead
Production in existing 2 ha	Unchanged (still capped)	Full intensive 60k
Time to ready	4–8 weeks (prep)	4–8 weeks (install)
Risk profile	Spread over more ponds, but 3× the area to manage remotely	Single site; battery must be live before relying on it
Flexibility	Locked into 36-mo lease	Asset you own; scales

Verdict: reject the pond lease as a power workaround. Renting the pond to dodge aeration costs ~₱24.62 per added kilo just in rent — over 6× the ₱3.99/kg that solar adds — and it locks you into a 36-month commitment while tripling the area a remote owner has to oversee. Neither option is ready for next week, so both are first-cycle build projects, not stocking-week decisions. Solar is clearly the better permanent fix. For next week, the answer is neither — stock 60k and manage DO with the 2 pumps + tidal exchange.

QUESTION 4 Financial comparison — 50k vs 60k vs 70k

Both ponds, 120-day intensive cycle. Total 4.5 ha — Box A (3.4 ha) + Box B (1.1 ha). Stocking split proportionally by area (Box A 75.6% / Box B 24.4%). Assumptions: fingerlings ₱1.75 each (5–7" size), survival 85%, harvest weight 420 g, FCR 1.9, blended feed cost ₱36.08/kg across the three feeds below, and ₱280K in other per-cycle costs (labor, inputs, pump power, misc).

Feed (current Bulacan, May 2026)	Bag price	Per kg	Protein
Starter	₱920 / 25 kg	₱36.80	31%
Grower	₱904 / 25 kg	₱36.16	30%
Finisher	₱894 / 25 kg	₱35.76	28%

Metric	50k	60k	70k
Fingerlings stocked (total)	50,000	60,000	70,000
Box A (3.4 ha) / Box B (1.1 ha)	37,778 / 12,222	45,333 / 14,667	52,889 / 17,111
Density (per ha, 4.5 ha total)	~11,111	~13,333	~15,556
Fingerling cost	₱87,500	₱105,000	₱122,500
Survival rate	85%	85%	85%*
Harvest weight (avg)	420 g	420 g	420 g
Total production	17,850 kg	21,420 kg	24,990 kg
Feed used (FCR 1.9)	33,915 kg	40,698 kg	47,481 kg
Feed cost	₱1,223,816	₱1,468,579	₱1,713,342
Other costs / cycle	₱280,000	₱280,000	₱280,000
Total cost	₱1,591,316	₱1,853,579	₱2,115,842
Breakeven price	₱89.15/kg	₱86.53/kg	₱84.67/kg
Gross revenue @ ₱100/kg	₱1,785,000	₱2,142,000	₱2,499,000
Net profit @ ₱100/kg (floor scenario)	₱193,684	₱288,421	₱383,158
Gross revenue @ ₱140/kg	₱2,499,000	₱2,998,800	₱3,498,600
Net profit @ ₱140/kg	₱907,684	₱1,145,221	₱1,382,758
Gross revenue @ ₱160/kg	₱2,856,000	₱3,427,200	₱3,998,400
Net profit @ ₱160/kg	₱1,264,684	₱1,573,621	₱1,882,558
Viable with 2-pump aeration only?	YES — wide margin	YES — with exchange	RISKY — heavy exchange*

* 70k assumes the same 85% survival only if the aggressive water-exchange protocol holds. A single missed pre-dawn intervention in a habagat week can drop survival sharply and erase the extra profit. The 70k profit number is real but conditional on near-perfect daily management.

Breakeven floor: You break even at ₱89.15/kg (50k) · ₱86.53/kg (60k) · ₱84.67/kg (70k). Even at a depressed farm-gate price of ₱100/kg — roughly 30–40% below normal Bulacan levels — every density is still profitable: ₱194K (50k) · ₱288K (60k) · ₱383K (70k). The farm is not wiped out by a bad-price quarter; it just earns less. At the typical ₱140–160/kg range, margins are strong across all three.

The money read: all three densities are comfortably profitable at every price shown — even the floor case (50k @ ₱100/kg) nets ~₱194K. Stepping from 50k to 60k adds ~₱95K at ₱100/kg, and ~₱309K at ₱160/kg, for a modest increase in management effort. Stepping from 60k to 70k adds another ~₱95–309K but the margin-of-safety on oxygen narrows from "comfortable" to "fragile." 60k captures most of the upside while keeping the farm out of the wipe-out zone. Cross-check against the full cost baseline in BFS-016.

TIMELINE Master schedule — stocking next week to harvest

Recommended path: 60,000 fingerlings, stocked June 2–4, 2026. Feed transitions and the critical monitoring dates:

Date	Day	Stage	Action / Feed	DO watch
Jun 2–4, 2026	0	Stocking	Acclimate & stock 60k at dawn; start Starter feed	Comfortable
Jun – early Jul	1–30	Nursery phase	Starter (31%); 5–12% daily exchange	~9.5 mg/L
~Jul 4	~30	Feed switch	Starter → Grower (30%)	Comfortable
Jul – early Aug	30–60	Grow-out	Grower; begin nightly 2-pump run	~8.6 mg/L
~Aug 3	~60	Monitoring step-up	Both pumps every night; daily 5 AM DO log	Tighten exchange
Aug – early Sep	60–90	Peak growth	Grower; 15–20% daily exchange	~7.0 mg/L
~Sep 2	~90	Feed switch	Grower → Finisher (28%)	Critical window opens
Sep – early Oct	90–120	Finishing	Finisher; 20–30% daily exchange; watch habagat nights	~5.6 mg/L — lowest margin
~Oct 1–4	~120	Harvest	Drain-harvest on ebb tide; ~21,400 kg	—

The danger month is roughly Sep–early Oct (Day 90–120). Biomass is at its peak, this overlaps the habagat (southwest monsoon) cloudy-day risk, and the pre-dawn DO margin is at its thinnest of the whole cycle. Every morning in this window: read DO at 5 AM, and if it is below 4 mg/L, increase the next tide's water exchange. This is the single most important operating discipline of the cycle — log it daily per BFS-017.

ACTION PLAN What Gary does this week

Confirm the stocking count at 60,000 fingerlings total — approximately 45,333 into Box A (3.4 ha) and 14,667 into Box B (1.1 ha), split proportionally by area. This is the recommended number — best balance of ~₱1.57M profit (@₱160/kg) and survivable oxygen margin with the 2 pumps you have. Place the fingerling order now (≈₱105,000 at ₱1.75/piece).
Verify both pumps are working and wired before the fish go in. They are your only aeration. Test-run both, confirm the line carries the load, and have a spare impeller/spare pump plan. A pump failure in September is a fish-kill risk.
Buy or confirm a DO meter and assign the 5 AM reading to the manager. The pre-dawn DO number is the single instrument that runs this cycle. No meter, no safe 60k — drop to 50k if a meter cannot be on-site by stocking.
Lock in the tidal water-exchange routine with the manager. Confirm gate operation against the BFS-014 tide calendar. The manager must be able to flush and refill on the daily tide — ramping to 20–30%/day by Day 90.
Order feed in phases, not all at once. Starter (31%) for Month 1, Grower (30%) for Months 2–3, Finisher (28%) for Month 4. Total feed bill ≈₱1.47M across the cycle — buy Starter now, stage the rest to match cash flow.
Treat solar as a first-cycle build, not a stocking blocker. Per BFS-018, the ₱855K solar system is the permanent fix — start sourcing quotes during this cycle so it's ready for cycle 2 and you can push to full intensive density safely next time. Renting the nearby 5-ha pond is viable as a capacity expansion (not as a power workaround), but costs ₱24+/kg in lease alone vs. solar's ₱3.99/kg. It makes sense only if the goal is to grow total production volume, not to solve the aeration problem.
Set the September review gate. When the fish hit Day 90 (~Sep 2), review the DO logs. If pre-dawn DO is consistently below 4 mg/L despite max exchange, plan an early partial harvest to cut biomass and protect the rest of the stock.

Bottom line: Stock 60,000. Run both pumps every night from Month 2. Manage oxygen with tidal water exchange, ramping to 20–30%/day in September. Read DO at 5 AM daily in the Sep–Oct danger window. Build solar during this cycle for a safer, higher-density cycle 2. Expected result: ~21,400 kg harvested in early October, ~₱1.57M net at ₱160/kg.