When tank comparison matters
Almost every beginner setup decision comes down to a comparison. 20-long or 29? 29 or 40-breeder? 75 or 90? Each pair has a different footprint, different height, different equipment cost, different stocking flexibility — and the difference isn't obvious from gallons alone. A 20-long and a 20-high are both "20 gallons," but the long version gives schooling fish ~50% more lateral swim space.
Which numbers actually matter
- Working volume.What you'll dose chemicals for. The number that drives stocking decisions.
- Footprint area. Two tanks with the same volume but different footprints suit different fish. Schooling tetras want length; angelfish want height; cichlids want footprint.
- Floor load. Past 500 lb, joist orientation matters and you should confirm the floor before placing the tank. The total-weight readout includes water + glass; furniture and substrate add more.
- Heater wattage delta. Heater costs scale with wattage, modestly. A 50W jump between tanks is ~$10 in heater, ~$2/month in electricity at 24/7 average duty cycle.
- Filter GPH range. Bigger tank → bigger filter, usually disproportionately bigger price. A 30g HOB is ~$30; a 55g canister is $80–120. Worth knowing before you commit.
- Weekly water-change amount. 25% on a 50g is 12.5 gallons of water carry-and-pour each week. Bigger tanks have nicer biology but more weekly chore.
Common comparisons worth running
- 20-long (30×12×12) vs 29 (30×12×18): same footprint, ~50% more volume, but the extra height is mostly useful for tall species (angelfish, gouramis). For schooling tetras, the long is usually the better pick.
- 29 (30×12×18) vs 40-breeder (36×18×16): slightly more volume in the breeder but much wider footprint (216 vs 360 sq in) — significantly more swim space and better for cichlids, larger schools, or aquascaping.
- 55 (48×13×21) vs 75 (48×18×21):same length, 5 inches deeper. The 75 is the right call if you're keeping anything beyond a small community — the 55's 13-inch front-to-back depth is famously cramped.
- 75 (48×18×21) vs 90 (48×18×24): same footprint, 14% more volume from the extra height. Marginal unless you have a specific tall species need; usually not worth the extra cost over a 75.
What this calculator doesn't tell you
- Stocking compatibility. A bigger tank is useful if your stocking plan needs it. Use the stocking density calculator to check that the species you want actually fit either tank.
- Equipment cost. The heater watts and filter GPH are sizing numbers, not price comparisons. Equipment cost scales nonlinearly — a 100W heater is ~$25, a 200W is ~$45, but a 300W is ~$80.
- Stand requirements.Once total weight crosses ~500 lb, off-the-shelf furniture cabinets are out and you need a purpose-built aquarium stand. The calculator flags the threshold but doesn't recommend specific stands.
- Hidden setup-time differences.A 75g cycle takes the same number of weeks as a 20g cycle, but the larger tank's water-change ritual takes longer once it's running. Plan around the maintenance cadence, not just the initial build.
Related
- Aquarium volume calculator — single-tank deep dive with the working-volume params (substrate, decor, fill below rim) /compare leaves at sensible defaults.
- Stocking density calculator — once you've picked a tank size, sanity-check the stocking you're considering against it.
- Heater sizing calculator — for cold rooms or warm-water species, the 68→78°F default doesn't apply.
- Water weight calculator — separate floor-load tool for second-floor placement on tanks above 75 gallons.