My 125 Gallon is already a so-called all-in-one system, as it lacks a sump. But it also lacks any extra filtration...no skimmer, no filter to run carbon, etc. So far it's been just live rock, LED lights and as much flow as I can give it.
This 125 was an upgrade itself almost a year ago.
I originally (11-ish years ago) had a 37 gallon+sump system for SPS...no fish.
After a few years I added a second display (50 Gallon) to that system for more coral space.
About a year ago all of the contents from both tanks were consolidated to the 125. I did have to add new lights to accommodate the increased tank length. The old sump and skimmer along with a little rock and the original sand bed were left behind.
I'm upgrading now for a few reasons.
I'd like to have the aeration from running a skimmer again. I have more algae growing that I have historically and I think corals are suffering from the day/night pH/oxygen swings.
I'd like the tank to get a little better flow from a lot fewer pumps. Currently there's a mix of six smaller Tunze powerheads.
I'm also upgrading the (still new) lights, but mostly for the superior form-factor. I like having excellent access to the tank for maintenance and the current lights are large for the space.
OLD
(all gear used at various times and listed roughly in order, ending with the most recent)
Lights: Coralife 2x 150 watt halide Radium, Ecoxotic, DIY, Maxspect Razor, Ocean Revive
Flow: Hydor Flo's, Hydor Korallia Gen 1 model 3 and 4, Seio Prop pump, Ecotech Vortech Gen 1 or 2, Tunze 6045 multiple generations
Return Pump: Quiet One 2200 (Italian), Mag 7, Quiet One 4000 (Chinese)
Skimmer: Oceanic "Plus Series" Model 6 (classic!!), AquaC EV-90, Tunze 9410
NEW
Lights: Three Kessil A360x's
Flow: Two Tunze 6105's
Skimmer: Tunze 9012
Also gaining a Tunze Osmolator ATO and Multifilter in the upgrade.
More info once I have the gear installed.