Fish Tank Tool is a simple tool for working out the volume of water held in an aquarium, its surface area, and the approximate number of fish that can be kept in it. Additional tools are included for calculating the dosage of medicines required for the volume of water in the tank, plus three useful conversions: between salinity as a percentage of sea water and specific gravity; between Celsius and Fahrenheit; and between centimetres to inches. These conversions are useful for making sense of books using unfamiliar units: for example European and Asian aquarium books use Celsius, litres, and centimetres; whereas American aquarists are more familiar with Fahrenheit, gallons, and inches. The application window is divided into 4 panels. The first, "Aquarium", is for calculating the volume of your fish tank. By default, the units are all set to the US system, but as soon as you change to the Metric system with this menu, the units throughout the application change accordingly. When you open the application, the pull down menu on this panel is labelled "--US--" and you can enter your own values of length, width, and depth in inches. Alternatively, you can choose "--Metric--" from this menu if you want to enter your own measurements in centimetres. Finally, you can use the values of the preset aquaria. The second window, "Fish", is for estimating the number of fishes you can keep, which depends on the surface area (not the volume) on an aquarium. Typically, aquarists recommend a ratio of one inch of small fish to every ten square inches of surface area (equivalent to 1 cm of fish per 25.4 sq. cm). Larger fish need proportionally more surface area. The surface area is given at the top of the window, and three buttons allow you to toggle between small, medium sized, and large fish. Medium sized fish need at least twice as much surface area per unit of length, and large fish at least three times as much. The third window, "Dosage", is for quickly working out how much medicine to add. Typically, aquarium medicines are dosed by units of medicine per gallon or litre, and this application does that calculation for you. Note that it actually doesn't matter what units you use: drops, capfuls, or teaspoonfuls will all work fine. You can either use the basic value, X units of medicine per litre (or gallon) of aquarium water, or a value assuming a ten-percent deduction from the aquarium volume for things like sand and rocks. The fourth window, "Convert", includes a variety of conversion tools. Typically, aquarium fish have their salinity preferences described in terms of percentages or fractions of normal seawater, for example 10% or 50% seawater. But aquarists cannot easily measure salinity, and instead measure the specific gravity of salty water to determine how salty it is. Normal sea water has a specific gravity of 1.023, and pure freshwater a specific gravity of 1.000. Drag the slider up or down the percentage scale to get the corresponding specific gravity. The other two conversion tools work in the same way, one for switching between Celsius and Fahrenheit and the other between centimetres and inches.