The two gallon systems differ enough to change equipment and volume comparisons.
One US liquid gallon equals 3.785411784 litres. One UK imperial gallon equals 4.54609 litres. The imperial gallon is roughly one-fifth larger.
One hundred litres is about 26.4 US gallons but only about 22.0 UK gallons. Both statements can be correct, which is why the word gallon needs a qualifier.
Original diagram supporting the us gallons and uk gallons are not the same explanation.
Common sources of confusion include US aquarium forums, imported products, traditional “10 gallon” tank names and pump or treatment instructions written for a specific market.
Convert the source figure to litres first, do the calculation in litres, then convert back only for comparison. This removes most ambiguity for UK users.
When a conditioner, treatment or equipment manual explicitly states US or imperial gallons, follow that definition rather than assuming the site’s default.
Label the unit as US gal or UK gal. A 100-litre aquarium is about 26.4 US gallons but about 22.0 UK gallons, so the ambiguity is too large to ignore when following equipment or product guidance.