Common reference numbers for freshwater aquarium setup — standard tank sizes, substrate depth guidelines, and filtration turnover rates in one page. Volume calculations use the exact US gallon definition of 231 cubic inches (NIST HB 44); liter conversions use NIST SP 811.
Standard freshwater tank sizes
Nominal sizes (5 gallon, 10 gallon, etc.) are manufacturer labels. Actual internal dimensions and true volume vary by brand and glass thickness. Gross volume is calculated from the listed dimensions at 231 in³/gallon.
| Nominal size | Typical dimensions (L × W × H, in) | Gross volume (gal) | Gross volume (L) | Footprint (sq in) |
|---|---|---|---|---|
| 2.5 gallon | 12 × 6 × 8 | 2.5 | 9.5 | 72 |
| 5 gallon | 16 × 8 × 10 | 5.5 | 20.8 | 128 |
| 10 gallon | 20 × 10 × 12 | 10.4 | 39.3 | 200 |
| 20 gallon (high) | 24 × 12 × 16 | 19.9 | 75.3 | 288 |
| 20 gallon (long) | 30 × 12 × 12 | 18.7 | 70.8 | 360 |
| 29 gallon | 30 × 12 × 18 | 28.1 | 106.3 | 360 |
| 40 gallon (breeder) | 36 × 18 × 16 | 44.8 | 169.5 | 648 |
| 55 gallon | 48 × 12.75 × 21 | 55.7 | 210.8 | 612 |
| 75 gallon | 48 × 18 × 21 | 83.5 | 315.9 | 864 |
| 90 gallon | 48 × 18 × 24 | 95.4 | 361.1 | 864 |
| 125 gallon | 72 × 18 × 22 | 129.1 | 488.7 | 1296 |
Measure your specific tank's internal dimensions for precise calculations. Manufacturers publish nominal sizes; actual capacity is ~5–10% lower after glass thickness is accounted for.
Substrate depth guidelines
Depth is measured from the tank bottom to the top of the substrate layer. These are industry-standard starting points from Aquarium Co-Op reference guidelines.
| Tank type / substrate use | Recommended depth | Approximate density | Notes |
|---|---|---|---|
| Fish-only, gravel | 1–2 inches | ~0.058 lb/in³ | Sufficient for aesthetics and beneficial bacteria colonization |
| Fish-only, fine sand | 1–2 inches | ~0.052 lb/in³ | Supports burrowing behavior for corydoras and other bottom dwellers |
| Planted, nutrient substrate | 2–3 inches | ~0.035 lb/in³ | Root depth for stem plants and carpeting species; lighter porous media |
| Cichlid / heavy digger | 3+ inches sand | ~0.052 lb/in³ | Allows natural substrate-sifting behavior |
| Bare bottom | 0 | — | Preferred for breeding tanks and some large predators; easier to clean |
Substrate depth requirements depend on plant species, fish behavior, and filtration strategy. These ranges are general guidelines — specific planted tank builds may require deeper substrate for root-feeding species.
Filtration turnover rate reference
Turnover rate is how many times per hour the total tank volume passes through the filter. This is a rough guideline, not a precise requirement; actual filtration needs depend on fish bioload, feeding frequency, and plant density.
| Tank type | Recommended turnover | Example: 55-gal tank |
|---|---|---|
| Low bioload (planted, few fish) | 4–6× per hour | 220–330 GPH filter |
| Community freshwater | 6–8× per hour | 330–440 GPH filter |
| Cichlid / high bioload | 8–10× per hour | 440–550 GPH filter |
| Saltwater / reef | 10–20× per hour | 550–1100 GPH total flow |
GPH = gallons per hour. Filter manufacturer ratings are measured at zero head pressure — actual flow through tubing and media is typically 20–30% lower than the rated GPH. Plan accordingly by selecting a filter rated above your calculated minimum.
Water volume vs. gross volume
Gross volume (from dimensions) overstates the actual water you need to treat and maintain. A typical freshwater setup with moderate aquascaping holds approximately 85% of gross volume as water. Use the full gross volume for filtration turnover calculations (filters are rated in GPH, not net-water-volume GPH), but use the ~85% figure for water conditioner dosing and water change volumes.
For calculating exact gross volume from your tank's measured dimensions, see the Aquarium Volume Calculator →. For calculating how much substrate you need by weight, see the Aquarium Substrate Calculator →.