I have order a sump system and 950 gphMonitor the Nitrates in the tank, your aim when doing a water change is 24 hours later to reduce them to half of what you tested prior to the change.
What filters are you using on the 180g tank. In my 75g I can get away with not doing a water change for 2 weeks, although I do a 50% change once a week. When changing water, you are not only helping to reduce nitrates but removing other unwanted things in the water. One being pheromones that the fish produce to potentially attract a mate.
In a large tank, you can slightly reduce the water change schedule but I would not do less than 50% per month.
Well yes and no. Thats waiting for your tests to show a number you dont like before you correct it.I read somwhere in the threads that water changes should be done as a result of water testing, not according to a time schedule. I think I agree,but what about those other things in water that are not measured by water test kits? Pheremones, stress hormones... . I think I'll keep to my weekly changes for now, but always learning more and willing to adjust to new knowledge.