1) I was wondering how can I know the exact power of the external filter I should be using for my aquarium? (aquarium dimensions:L=150cm, H=105cm,W=28cm)
2) Can I use a mirror as a background?
There are many types of filters in the market.
You can get the local pet shop people to help.
Placing a mirror at the back as a background is wonderful.
It would not disturb the fish.