200 watts is a pretty powerful heater and may be sufficient. While larger units are often recommended for big tanks such as your 125, it really depends on the "differential" you have to work with - what your room temperature is and what temperature your fish need.
Usually, around 75F~24C is a good starting point, but some fish will need it a little warmer, while others will need to be a little cooler.
What fish do you plan to keep? You temperature will depend on your choices. Look at care sheets for your desired species and see what their requirements will be.