Add 100 mg of drug "A" to a fish tank filled with water. Take a sample after mixing is complete. Result: The measured concentration of the drug in the water is 10 mg/l (10 μg/ml). This example assumes that the drug remains in solution in the water and is not attracted to glass (gravel, decorations, plants, etc.).
Determining the volume of a fish tank by adding a substance and measuring its concentration.
The "apparent" volume of the tank is calculated as:
Vz = Amount ÷ concentration

Vz = 100 mg ÷ 10 mg/liter = 10 liters 

Add 100 mg of a drug "B" to the same 10 liter fish tank filled with water. Take a sample is taken after mixing is complete and equillibrium with other aquarium components is reached. This example assumes that the drug leaves the water because it binds to glass (gravel, decorations, plants, etc.).
The "apparent" volume of the tank is calculated as:
Vz = Amount ÷ concentration

Vz = 100 mg ÷ 1 mg/liter = 100 liters 

Determining the "pharmacokinetics" of a fish tank. Units in this example assume that the "fish tank" represents one kilogram of body weight. The dose of drug is 100 mg/kg, the volume of distribution is 1 liter/kg. Values for organ flow (Q) and extraction efficiency (E) are characteristics of a specific filter hung on the tank. Samples are collected at the times reported in the figure and concentrations of drug are determined as the filter clears the drug from the tank.

Drug concentrations vs time
Arithmetic plot
Semi-Logarithmic plot
Pharmacokinetic constants for this fish tank experiment are:
lz = 0.0693 hrs-1,  Vz = 1 liter/kg, Dose = 100 mg,  T1/2 = 10 hrs, Clt = 0.0693 liters/kg/hr