I've got a 40 gallon system II. I'm pretty happy with it...but I modified it. I took out the bioballs and tray and utilize that area with mangrove plants. I currently don't use a skimmer. When it says 100 gallon...you are not getting 100 gallons of swim space...it's just the external dimensions of the tank. I'd estimate that my sump/filter area is about 8 gallons on my 40 gal
. Here's what I see as Pros/Cons:
1-Filtration is pretty good. You could theoretically go without powerheads in the tank as the manifold makes an L in the back of the tank and covers a good area.
2-Decent size filtration box to use as a refugium or for live rock rubble
3-Attractive tank. But I'm partial to acrylic.
4-You can hide pretty much everything in the box...ie heaters, chemical media to keep the display tank pretty natural looking
1-If you like to do things your way, you are limited by the setup in this tank. Basically if your pump fails, you'll just buy a new one and keep the setup the same way.
2-Keeping sand is difficult. Give up on a deep sand bed with this thing as the way the manifold is setup it's a sandstorm along the bottom of the tank and you'll never have a level sandbed. I keep a 1" sandbed in my tank and I can keep it looking pretty good as long as I have rock blocking some of the manifolds.
3-Space. Like I said. You would not be getting a 100 gallon swim tank...more like a 90 gal
4-Acrylic scratches easily. I have a puffer in mine and his teeth scratch the acrylic...no big deal tho as scratches can be removed.
Would I get one again? Hhhmmm...not sure at this point as I was a newbie when I bought it and think as time goes on I'd like to mod my own tanks as I see fit. I would definitely consider an empty Seaclear tank though. Does it do a good job of filtration? Yes it does! I've never had any stability problems with the water and even with a larger bioload. If you get it, don't use the bioballs and tray and get some mangroves off of ebay...and cycle the tank with some raw shrimp. If you are a newbie, it's a good way to go and get introduced to saltwater as it takes alot of the guess work out for your first tank. It's really a simple tank to maintain and all self-contained as well...I've got my 40 gallon on a heavy mexican style trunk and it's a nice tank to look at. I don't have to worry about a tank stand and someplace to hide an external sump or refugium.
If you have any other questions, either post here or PM.