Everyone has there opinions and none are really wrong. This is what I use on my 180g which houses two large silver arowana(21-24") and another 17 4-5" fish. I change 50% of the water weekly.
I have 2 AC110s and a xp3. The sponges of the ACs are rinsed weekly in hot tap water and do a get job for mechanical filtration. The xp3 is packed with good quality bio-media and has yet to fail to keep the water parameters in check. This set up is about the most inexpensive setup you can go with and is very efficent.