Having run a pair of IM Lagoon 25s for close to a year, I thought I’d pass along a few tips that I learned. Apologies in-advance for the small wall of text...
First and foremost, ditch the IM UV sterilizer (or at least only use it when introducing new livestock). The black sponge it comes with is ill-suited for any serious kind of long-term filtration. If you do use the IM UV (again, in short durations), I recommend a thin layer of filter floss cut to fit over the black sponge (that way, you can just toss out the filter floss every few days and never worry about also needing to clean the black sponge). And you'll keep the UV tube relatively crap free - so it will be more effective.
While you may tempted to run two IM caddys - don't. Believe it or not, your most successful nitrate control method are those pesky filter socks. Removing them every few days when they start to overflow actually gets rid of the majority of any nitrate buildup. You will want to invest in a good half dozen desktop size filter socks as well. So caddy w/filter media + sock filter = your best method of keeping the tank clean.
The stock IM caddy has pros/cons. The cons are that it doesn't provide a tight seal and you will find a lot of water simply bypasses under and around the lip when it's in-place. The pro is that if it gets clogged it has a slot at the top which allows the water to bypass the caddy and not cause the tank to overflow (similar to the sock filter). I believe the IM tanks also come with a caddy. If you want an awesome upgrade I recommend the InTank caddy (but this is something you can always look at down the road). It's angled and provides a much tighter seal than the stock IM caddy. The downside is that there's no overflow option, so you need to run a sock filter to ensure your tank doesn't overflow in the display area.
With respect to filter media, stick with ChemiPure Blue (never, never ChemiPure Elite) and filter floss in the top. You'll need to remove the middle shelf to fit the whole bag in, and since it's probably rated for 35 gallons you should be good without a change for 2-3 months (depending on your bio load).
Now for a downside. The IM Lagoon 25 has (IMHO) a massive design flaw in the AIO design. Remove the pump in the center rear chamber, reach your hand in and feel along the right and left sides. Notice a gap (about 1" high) that runs front to back? The side filter chambers don't run to the bottom of the tank - they only extend down to about 1" or so from the bottom. So there's a 6-8" gap on either side of the rear pump chamber that you'll never be able to access or otherwise clean. But it holds water, and thus all sorts of detritus and a great hiding spot for anything that ever jumps or climbs into the sump (oh yeah, fun times!) I didn't realize this when I filled my tanks with water, but had I known beforehand I probably would've cut some acrylic and siliconed those gaps shut. I'm sure IM did it this way so you could have a bit more water in the pump chamber, but I'm not sure that slight benefit is worth the stagnant water that accumulates there.
I tried MarinePure bio balls (hexes) in the back sump and I just don't think the AIO design has enough of and/or proper flow to be effective. They just seemed to accumulate detritus like the plague and contributed rather than decreased the amount of cleaning in the sump areas. Some people have had good success running refugiums (InTank also makes a special refugium basket for the IM 25 Lagoon), but just keep in mind that due to the lack of water flow in the second chamber it tends to "gunk up" fairly quickly.
Upgrades. A pair of IM SpinStream nozzles was a great addition and helped eliminate dead spots in the tank. If you opt for these do note that they need to be removed and thoroughly rinsed/scrubbed out every few months as they do accumulate slime (or just throw them in a small container of Ezeclean overnight and rinse out the next day). The wavemaker will provide most of your flow anyway. You're opting for a non-IM skimmer so you're already saving yourself hours of mess from overflowing container cups and pumps that breakdown every 2-3 months. I'm not sure how effective the IM reactor is, but I never had any luck reducing nitrates or phosphates with it. In theory you could cram a ChemiPure Blue bag into it if you wanted to go that route.
You'll want to invest in some sort of auto top-off system. You will (no exaggeration) be literally topping this up daily with about 3-4 liters of RO water. It doesn't sound like a big deal at first until you decide to go away for the weekend and realize your tank is going to go dry by the time you check into the hotel... You can minimize this to some extent with a solid lid, but this also cuts back on your oxygen diffusion and adds some major salt creep to the sides and lid that will up the maintenance.
You can definitely run two heaters, and your setup will be fairly bulletproof (and give you some remote monitoring through the Inkbird). Do keep in mind though that tanks can withstand a prolonged temperature drop - it's the lack of flow and oxygen within a few hours that ultimately wrecks them. Putting your wavemaker on UPS or battery backup to get you through a short power outage is something I'd recommend as a higher priority.
