Yes you can over skim but not in the most think. If you take that 200 and put it on your system that you have now, what will most likely happen is it will skim and produce foam as normal and then it will just stop producing foam at one point as it has nothing it can remove. Now this can last sometimes for a week and then you would see it would start once again, foam and the process starts over and repeats itself. The skimmer does not produce a consistent foam, some say no big deal. IMO I believe it is, as the organics bounce up and down at much greater rates than if it was a consistent foam. The Organics would spike and fall, so let's just use a very basic sort of example. Say you have a base number ( FICTIONAL ) that a skimmer keeps your tank happy and the skimmer behaves it a consistent manner, of say 25 ppm. Now you get a larger unit that is well oversized and now it starts to work, for a day or two then it just stops producing, levels have drop to say 5 ppm, threshold point. Now the Organics have to build up in your system before it will start again, but now yes it will start to foam at 25 ppm but organics are on the rise and it can't stop at 25 ppm because the Organics have built up and the skimmer does react to the increase but it does take time for it to remove, the levels increase to a max say 150 ppm ( at this point most start posting and saying it's the best skimmer it is foaming like a beast ) before they start to fall back down until it bottoms out again at 5 ppm ( at this point you see postings saying skimmer sucks and just stopped and I have to adjust all the time and then it will overflow and I am sick of adjusting it it sucks, I need a new skimmer any ideas on a good skimmer ). Now take that Elite 200 and put it on the 180 and say it's happy holding around 25 ppm, you feed and organics go up and say rise to 50 ppm and brings it down back to around the base but not below. It is now producing a consistent foam. Now in Reefing one of the most important things is being consistent with water parameters and I believe going with the seesaw method can cause more problems then solve. The point I am trying to make is Skimmers are much more efficient now than they were when that double or triple the size or double the skimmers rule was suggested, this goes back to the Air Driven units, you may see some of them for sale today for Nano tanks. I believe Lees and Reef Octopus still sell then and I sure some others. IMO you want to look for a skimmer that the manufacture has rated your system somewhere in the middle. Example your future 180 ( don't forget you need to base it on tank Volume once everything is in it and the sump, so potentially your 180 may end up with total volume of 120 after Rock work and substrate ect. ) Skimmer rated 75 gal Heavy and Mid 150 and 200 light is a better choice than 150 Heavy 225 Mid and 300 Light. Like it was stated before a max of 30 to 35% is what they can remove. GAC will remove much greater amounts anywhere from 60 to 90%, another reason to think do I need a large skimmer. Like many people say " I added Carbon and now my skimmer has stopped or almost stopped working."
Just my 2 cents and maybe something to think about.
