Why a 20-gallon tank holds fourteen gallons
Tanks are sold by a nominal size that comes from their outside dimensions. A 20 high is 24 by 12 by 16 inches, which is 4,608 cubic inches. A US gallon is 231 cubic inches by definition, so that is 19.95 gallons, and the name is honest as far as it goes.
What goes in is another matter, and it comes off in three steps.
| Step | 24 x 12 x 16 in tank |
|---|---|
| Outside dimensions | 19.9 gal |
| Less 1/4 in glass all round | 18.4 gal |
| Less 1.5 in of freeboard | 16.7 gal |
| Less 2 in of substrate | 14.3 gal |
That is 72% of the number on the box. Most of the time the difference is harmless. It stops being harmless when you dose medication, because a dose measured for 20 gallons going into 14.3 gallons arrives 40% stronger than intended, and the safety margin on copper-based and formalin-based treatments is not 40% wide. The same error runs through fertiliser dosing, salt for a freshwater dip, and any water change percentage you are trying to hit.
Why the substrate is subtracted whole
A substrate bed is not solid. Sand and gravel are somewhere near 30% void by volume and that void is full of water, so subtracting the whole bed removes water that is genuinely in the tank. The calculator does it anyway, and on purpose.
Consider which way you want to be wrong. If the stated volume is too high, every dose you calculate is too concentrated. If it is too low, every dose is slightly weak. Weak medication means treating for another day. Concentrated medication means dead fish or a crashed filter. So the number is biased in the safe direction and the bias is stated rather than hidden. If you need the true figure for something else, add back about 30% of the substrate volume shown.
Hardscape works the other way and can be measured exactly: whatever water it displaces is water that is not in the tank. Fill a bucket, mark the level, put the rock in, and the rise is your answer. Porous lava rock and dry driftwood both absorb water over the first weeks, so their displacement drops slightly once they are saturated.
Turnover, and what it does not tell you
Filters are rated in gallons per hour, and dividing that by the tank volume gives the turnover multiple. Fourteen gallons through a 70 gph filter is five times an hour.
The conventional targets are around three times for a lightly stocked planted tank, five for a normal community, eight for goldfish or heavy feeders, and ten or more for large messy fish and turtles. Those are starting points. Two things bend them badly. The first is that catalogue ratings are measured with an empty filter body and no lift, and a filter packed with media pushing water up two feet delivers something closer to two thirds of its rating, which is why the calculator also shows the rated figure to shop for. The second is that biological filtration is a function of media surface area and the oxygen reaching it, not of how fast water moves past. A large filter run gently outperforms a small one run hard, and past a certain flow the fish spend their energy swimming rather than eating.
Weight is the part people find out about late
Water is 8.345 pounds a gallon and substrate is heavier than water. The example tank above holds 14.3 gallons of water at 119 pounds, plus 2.3 gallons of sand at about 29 pounds, for 149 pounds of contents in a footprint of two square feet. Add the glass and the stand and it is comfortably over 170 pounds standing on 24 by 12 inches of floor.
Residential floor framing is designed around a 40 pound per square foot live load, but that figure is an average across a span rather than a limit at any one point, and a filled aquarium sits in the same place for years. Small tanks are a non-issue anywhere. From roughly 55 gallons upward, position starts to matter: against a load-bearing wall rather than mid-span, with the long axis crossing the joists rather than running parallel to a single one. In an older building, or over a basement you have not looked at, the honest answer is to look at the framing before you fill the tank rather than after.
Questions people ask
Should I dose medication by the real volume or the tank size?
The real volume, every time, and it is the reason this calculator exists. Dosing a 20-gallon tank that holds 14.3 gallons of water at the label rate for 20 gallons puts the treatment in 40% over strength. Some medications tolerate that and some do not — copper-based treatments, formalin and anything aimed at parasites tend to have narrow margins, and invertebrates and scaleless fish have narrower margins again. The same logic applies to fertiliser, dechlorinator and salt. When a treatment is going in at full strength for several days, being deliberately slightly under is the sane place to sit.
How much water should I change and how often?
Twenty-five to fifty percent weekly covers most freshwater community tanks, and the calculator converts whatever you pick into gallons and into inches of water level so you can mark the glass and stop measuring. What actually decides the number is nitrate. Test before a change and test again a week later; if nitrate is climbing week over week, you are changing too little or feeding too much. Match the temperature of the new water and dechlorinate it before it goes in. Very large changes in one go are not automatically bad, but they swing temperature and pH more than small frequent ones, which matters more in a tank with sensitive fish.
Is one large heater or two small ones better?
Two smaller ones, in most tanks over about 30 gallons. Heaters fail in two ways: off, which gives you a slowly cooling tank and time to notice, and stuck on, which is the one that kills fish. Two heaters each sized at about half the requirement mean a failure-off leaves the tank warm enough to survive, and a failure-on can only push the tank as far as half the wattage manages, which in most rooms is a few degrees rather than a boil. It also spreads the heat, which helps in a long tank. The cost is a second heater in the tank and a second thing to fail, which is why below 30 gallons a single heater plus a thermometer you actually check is usually the better trade.
Does the calculator work for a rimless or bow-front tank?
Rimless, yes, with one adjustment: you can normally run less freeboard because there is no plastic rim eating the top inch, so drop that field to half an inch or so unless a hang-on filter needs more. Bow fronts, not exactly. The curved front adds volume the rectangular formula does not see, so the figure here reads low by roughly three to eight percent depending on how pronounced the bow is. If that matters, measure the tank against a known volume instead: fill it from a container of known size and count. Cylinder mode covers round tanks properly, and hexagonal or other odd shapes are outside what this handles.