PS. You aren't using correct measurements, milligrams is the measurement of mass while gallons is the measurement of water volume, giving us the mass won't help determine the water volume, from the visual picture and the size of the Betta/decorations, it does look slightly smaller than a gallon
are you sure your 10 years old? because i really don't believe it
Milligrams, a unit of mass, and gallons, a unit of volume, are not actually directly equivalent, however it is common to use the volume and mass of water to "equate" the two.
Using that method, where
1 (U.S.) Gallon of water weighs 128 ounces, and
1 ounce weighs 28.35 grams (approx.) or 28,350 mg
1 Gallon "equals" 128 * 28,350 mg, or 3,628,739 mg
Yes, the measurement 1mL = 1mg is for pure water only, after that you get into decimals and percent composition of the water and such. We use it all the time in our chem conversions.
However it is close enough to be terribly worried for the little guy.