Before water goes in
The setup math. Run these once, while the tank is still empty on the stand. Wrong numbers here cascade into every downstream decision — dosing, stocking, even whether the floor is rated for the finished weight.
Aquarium volume calculator
- Use this when
- You bought a tank labeled 'X gallons' and need the real working volume — for dosing, stocking, and water changes.
- What it tells you
- Inside-dimension US gallons and liters for rectangular, bow-front, cylindrical, and hex tanks, plus working volume after substrate + air gap.
- What it can't see
- Heavily-aquascaped hardscape volume — drain-and-measure beats any model on a packed planted tank.
Method & sourcing
Inside-dimension geometry ÷ 231 cubic inches per US gallon (NIST exact). Bow-front uses a half-elliptical bulge; cylinder uses π·r²; hex uses (√3/2)·F². Saltwater is ~2.5% denser; weight readout is freshwater-tuned.
Water weight calculator
- Use this when
- You're planning a tank above 75 gallons or putting it on an upper floor and want to know what the slab will carry.
- What it tells you
- Total filled weight in pounds and kilograms — water plus a typical estimate for glass, stand, and substrate.
- What it can't see
- Specific local floor rating — for upper-floor tanks past 100 gallons, run the number past a structural engineer, not the internet.
Method & sourcing
8.34 lb per US gallon for freshwater plus structural mass estimates from Aqueon / Marineland spec sheets. Saltwater bumps water density ~2.5%.
Substrate calculator
- Use this when
- You're at the fish store deciding how many bags of sand, gravel, or aqua-soil to grab for a given footprint + bed depth.
- What it tells you
- Pounds and gallons of substrate for your footprint, with 10% padding so you don't end up one short.
- What it can't see
- Brand-specific bulk density — Eco-Complete weighs differently than play sand. Padded the result to absorb most of that drift.
Method & sourcing
Footprint × depth converted to volume, then mass via material density. Sand 95 lb/ft³, gravel 100 lb/ft³, aqua-soil 60–70 lb/ft³.
See alsoCompare two tanks side-by-side — live deltas on volume, weight, heater wattage, filter GPH, and footprint. Helpful when you're deciding between a 55-gallon and a 75-gallon on the same stand.
Before fish go in
The decision layer between an empty cycled tank and a stocked one. Run cycling math first, then the stocking checkers. Adding fish before the cycle is one of the mistakes that creates most new-tank emergencies — it's also the most common.
Fishless cycling calculator
- Use this when
- Empty new tank. You want to cycle without subjecting fish to ammonia spikes.
- What it tells you
- Day-by-day ammonia dose to seed the bacterial colony and a stage classifier from each test reading — when 'ammonia + nitrite both drop to zero in 24h' means fish-ready.
- What it can't see
- Live-plant or seeded-media speed boosts beyond a rough estimate — actual stage depends on test readings, not a calendar.
Method & sourcing
Targets the 2–4 ppm seed range (above that suppresses bacterial growth). Pure ammonia and Dr. Tim's chloride math. Stage transitions follow the standard Nitrosomonas → Nitrospira sequence in mainstream cycling references.
Compatibility checker
- Use this when
- You've got a specific list of fish in mind. You want to know whether THIS exact mix lives together.
- What it tells you
- A verdict on parameter overlap, predator-prey risk, schooling minimums met, tank-size minimums met, aggression conflicts, plus three species that would complete the stocking.
- What it can't see
- Individual fish personality — a single rogue tiger barb voids the model. Use the verdict as the conservative floor, not a guarantee.
Method & sourcing
Pairwise checks against FishBase temp/pH/GH ranges, body-size predator math, hobby-consensus schooling minimums (Aquarium Co-Op + Tropical Fish Forums + Practical Fishkeeping), and adjusted bioload.
Stocking density calculator
- Use this when
- You want to know whether a proposed stocking is at, near, or past the tank's biological ceiling.
- What it tells you
- Safe / caution / overstocked / incompatible verdict plus the bioload + space-minimum reasoning. Not inches-per-gallon — that rule is wrong.
- What it can't see
- Filtration-driven overstocking headroom in heavily-filtered tanks. The model assumes typical HOB or sponge filtration.
Method & sourcing
Adjusted bioload by body shape + activity level + temperament. Tank-size minimums from species cards. Schooling floor checks run alongside.
Equipment sizing
Heater wattage, filter flow, and air-pump output you actually need. Rules of thumb (5 W per gallon, 10× turnover) under-spec winter heaters and over-spec planted-tank flow. The math here is from manufacturer sizing tables, not forum lore.
Heater wattage calculator
- Use this when
- You're sizing a heater and the box recommendations bracket your tank between two wattages.
- What it tells you
- Recommended wattage based on tank volume and the temperature delta between your room and target tank temp.
- What it can't see
- Drafty room corners, basement winter dips, glass-top vs open-top heat loss — buy one tier up if any of those apply.
Method & sourcing
Cross-checked against published manufacturer sizing tables (Aqueon, Fluval, Eheim). Conservative bias toward the next wattage up.
Filter flow rate calculator
- Use this when
- You're shopping for a filter and need to translate 'box GPH rating' into 'actual working GPH for my stocking.'
- What it tells you
- Recommended filter GPH range, with the rated-vs-actual flow gap baked in so the box number you shop for matches the working number.
- What it can't see
- Media-clogging drop-off over months — clean filter monthly to stay near the working number.
Method & sourcing
10× turnover floor (Aquarium Co-Op consensus) with stocking-density and tank-shape adjustments. Rated-to-actual penalty from manufacturer disclosure.
Air pump size calculator
- Use this when
- You're adding a sponge filter, undergravel, or airstone and need to pick a pump that actually drives it at your tank depth.
- What it tells you
- Required L/min plus SKU references, with a depth-pressure flag for tanks past ~18″.
- What it can't see
- Stacked tank rack pressure loss — if you're running one pump across multiple tanks, size up generously.
Method & sourcing
Manufacturer L/min tables (Tetra Whisper, Hygger, Aquatop) mapped to filter mode and tank depth. Depth past 18″ adds back-pressure that derates many small pumps.
When test results look bad
Emergencies. Acute toxin readings or visible distress (gasping, lethargy, dead fish in the morning). Start with the tank problem, then pick the tool that matches what your test kit is showing.
Ammonia / nitrite emergency calculator
- Use this when
- Your test kit reads non-zero ammonia or nitrite and fish are still in the tank.
- What it tells you
- Exact water-change percent and Prime/dechlorinator emergency dose to bring readings back to fish-safe. Decision tree by severity tier.
- What it can't see
- Whether the cycle has crashed permanently or just stalled — re-test after the staged change to find out.
Method & sourcing
Targets ammonia under 0.25 ppm. Staged-change math avoids the pH/temperature shock that a single huge change introduces.
Nitrite spike calculator
- Use this when
- Nitrite is high and you want the targeted salt dose that protects gills while the cycle finishes.
- What it tells you
- Water-change percent plus a targeted aquarium-salt dose to block nitrite uptake at the gill (chloride competition mechanism). Salt-sensitive-species aware.
- What it can't see
- Plant tolerance of the heavier conservative dose — toggle salt-sensitive ON if you have shrimp, snails, corydoras, kuhli loaches, or stem plants.
Method & sourcing
10:1 chloride-to-nitrite mass ratio (aquaculture standard, Boyd) for the targeted dose; 1 tsp/5 gal for the heavier hobby-standard dose. Salt does not remove nitrite — water changes do.
Fish gasping at surface — diagnostic
- Use this when
- Fish are gasping at the surface and you don't yet know whether it's ammonia, nitrite, low oxygen, pH, or disease.
- What it tells you
- A ranked diagnosis from your test readings + tank conditions. Top suspect first, others ranked after — multiple causes can present at once.
- What it can't see
- Disease pathogens — for clean test results + visible gasping, the calculator falls back to disease and points you to a vet-staffed forum.
Method & sourcing
Severity-weighted ranking: toxin readings outrank ambient conditions, ambient conditions outrank disease. Each cause has its own action checklist.
New tank syndrome calculator
- Use this when
- Tank is under 8 weeks old, you have fish in, and readings are off. You want to know where you are in the cycle.
- What it tells you
- Cycle stage from your three test readings (pre-cycle, ammonia-spike, nitrite-spike, near-cycled, cycled) plus estimated days remaining and the action steps for that stage.
- What it can't see
- Crash-and-restart from a filter-clean accident vs an in-progress first cycle — pattern is similar, history is the differentiator.
Method & sourcing
Pattern match on ammonia + nitrite + nitrate readings against the standard Nitrosomonas → Nitrospira sequence. Live plants shorten the estimate by ~1 week.
Weekly care
The math you run every Sunday. None of these are emergencies — they're the routine maintenance that keeps emergencies from happening in the first place. Once a tank is cycled and stocked, this lane is most of what 'keeping fish' actually is.
Water change calculator
- Use this when
- You're setting up a routine maintenance schedule and want to know per-session volume + hauling weight + cadence.
- What it tells you
- Per-session gallons, hauling weight, weekly + monthly cadence, and a verdict on whether the schedule is conservative-safe or recovery-only.
- What it can't see
- Whether your specific stocking actually needs more than the entered percent — pair with the nitrate target mode to find out.
Method & sourcing
25–30% weekly is the conservative hobby baseline. Nitrate-target mode uses mass-balance math accounting for tap-water nitrate.
Dechlorinator dosage calculator
- Use this when
- Water-change day. You want the exact Prime / Stress Coat / AquaSafe / Fluval Aqua Plus dose for the new water going in.
- What it tells you
- Per-product dose math from manufacturer spec sheets. Doses the NEW water added, not the whole tank.
- What it can't see
- Whether your tap is chlorine or chloramine — most modern dechlorinators handle both, but check the bottle. Older single-purpose chlorine removers do NOT bind chloramine.
Method & sourcing
Manufacturer label rates: Prime 1 cap per 50 gal at 1×, Stress Coat 5 mL per 10 gal, AquaSafe 5 mL per 10 gal, Fluval Aqua Plus 5 mL per 38 L.
High nitrate calculator
- Use this when
- Nitrate is climbing past your target between water changes and you want to know how much extra to swap out to recover.
- What it tells you
- Water-change percent by tank context (community 40 / shrimp 20 / breeding 10 ppm targets) and whether the target is even reachable given your tap nitrate.
- What it can't see
- Source-water nitrate that exceeds your target — calculator flags this as 'unreachable' and points you toward RO or nitrate-binding media.
Method & sourcing
Mass-balance dilution math. Above 50% it stages: 50% now, retest in 24 hours, repeat. Tap-water nitrate is the hard floor.