So, i am moving my rats from their 20L tank into a wire cage and decided i wanted to make that tank into a fish tank. I have never had one larger than a fish bowl, and need some help. What is the best kind of filter? water temps?

i just need to know some general things before i put a bunch of water and fish in there.

Thats another question...i don't want to mess with salt water, what are some cool fish that i can do in fresh?

Thanks everybody!