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Stocking Calculator

Two completely different problems live under "stocking". Freshwater has a defensible heuristic (inches of slender-bodied fish per gallon, with a surface-area kicker) that works for small community fish. Saltwater has no formula — only species behaviour and territory rules. This calculator gives a useful answer for the first and a deliberate non-answer for the second.

Stocking

Stock list

Columns: species, adult length (in), bioload index, count.

Bioload points
6.0points
Recommended ceiling
18.8points

~0.25 points / US gal

Utilization
32%
Status
OK
  • ⚠ Saltwater stocking has no universal formula. This is a qualitative bioload heuristic — read the WetWebMedia stocking FAQs and stock by species behaviour, not formulas.

How this is calculated

Freshwater

surface_factor = (length × width) / 432   // baseline 36 × 12 in
effective_gal  = tank_gal × surface_factor
inches_per_gal = sum(adult_inches × count) / effective_gal
status         = OK if ≤0.75, Watch if ≤1.0, Overstocked if >1.0

The reference surface (432 in²) is the footprint of a 'classic' 30 gal where the inch-per-gallon rule was implicitly calibrated. Bigger surface = more O₂ exchange = more livestock supported per gallon. The rule breaks for fish with stocky bodies (cichlids, goldfish, plecos) or adult sizes above ~6 in — both flagged.

Saltwater

bioload_points = Σ bioload_index × count × max(1, adult_in / 3)
ceiling        = 0.25 × tank_gal       // heuristic — no primary source

The 0.25 points/gal ceiling matches WetWebMedia-style consensus that a 100-gal mixed reef holds maybe two tangs and a few smaller fish at adult size. It's a heuristic, not a constant. Use it as a sanity check, then read the WetWebMedia FAQs and species-specific guides before adding anything.

FAQ

Why is there no saltwater inches-per-gallon rule?
Marine fish have radically different territory and behaviour requirements that don't scale with body length. A 4-inch tang needs more swim length than a 4-inch eel needs hiding holes. Two ocellaris clownfish are fine in a 20 gal; two yellow tangs in the same tank end with one dead tang.
Does the bioload index account for filtration?
No. This is a stocking sanity check, not a filtration model. Adequate skimming, mechanical filtration, biological media, and water changes are assumed. A 5-point bioload doesn't magically work in 20 gal because you added a bigger skimmer.
What changes if I have a sump?
Add sump water volume to your tank volume — that's all sump filtration is from a bioload standpoint. The real benefits (heat dissipation, surge buffering, equipment hiding) don't show up in stocking math.
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Sources & references

  1. 01
    WetWebMedia — Stocking FAQ (Bob Fenner / Neale Monks)
    http://www.wetwebmedia.com/ca/volume_5/volume_5_3/stocking.htm
  2. 02

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