no because if its a freshwater tank the rain has salt in it
This is not advisable as composition of rain water is nowadays unpredictable.
An intense bushfire would severely warp a plastic rainwater tank.
A heater can raise the temperature of a fish tank if needed
Rainwater is distelled water and tankwater is used for home porpes
Rainwater litter is probably the little bits of leaves and dirt that collect on your roof and are washed down with the first flush of rain.If you have a rainwater tank you should consider having a "first flush diverter" or a "rainwater litter catcher" so that the litter does not end up in your tank.
The mass of a fish tank should be measured in kilograms.
depends on the size of the fish, there might not be enough oxigen in a small fish tank.
The simple answer is yes.
math
Of course you can.
It will either have to get used to it or die.
No