You don't really need to measure the water in a birdbath, just put some in a couple inches deep.
The number of liters in a pond depend on the size of the pond.
A pond would be measured in litres.
A glass tank.
Yes
The balloon
You lash fish to a giant hamster wheel. Then you attach the axle to a generator. The power transferred is dependent on the size and amount of fish.
The location of the fish in the water is not where it seems to be, due to the refraction of light in the water.
A fish tank would hold about 6 liters.
The unit typically used to measure the amount of water in a fish tank is liters (L) or gallons (gal).
The official SI unit for volume is the cubic meter, but for a fish tank, I would use liters. One liter is one cubic decimeter (or 1/1000 of a cubic meter).
If the fish does not struggle (it would have to be dead not to) and above water, then you can measure a spring scale.
the fish tank
Litres. (L)
About 2,422 liters.
80 liters will fill a good sized fish tank [about 21 gal]. 80 milliliters would hardly start, as that is about 5 tablespoons.
fish tank
Measure its dimensions in inches or centimeters, and its volume or capacity in gallons or litres.
a meter stick.
Litres.