Fish Tank Tool performs a variety of tasks such as calculating the volume and surface area of an aquarium, estimating the number of fish that can be kept in it safely, and quickly converting between different units of temperature, length, and salinity. It is designed for aquarists keeping freshwater and brackish water aquaria.
NEALE MONKS A request for a metric version was made some other users as well, and you'll be pleased to know there are now US and Metric versions for both Mac and Windows: