if you get another make sure that you have filtration for at least twice as many gallons that are in your tank. they are very dirty animals, so for your 25 gallon tank, you need a filter for 50 gallons or more.

time of introduction depends on the size of your frog, and the size of your new frog, you want them to be roughly the same size so they dont eat one another, which has been known to happen often if given the chance.

my current frog now, oliver, is roughly 1 1/2-2" long. i got him out of a tank with his sisters, and they are bigger than he is. which i know is normal for females to be larger. but i was thinking about breeding so i wouldnt want to get his sisters. do you think that if i got a frog about a year from now it would be ok?

as long as they are the same size... yes

what is the biggest size difference you can have before the larger eats the smaller?