Welcome to R2R! Glad you found us.
There are a few options for adding a fuge to the 525XL sump that I've seen (and tried);
1) Convert the ATO to a fuge (using an external ATO tank in the right cabinet or outside the stand somewhere). This is nice because it keeps the fuge in the stand and uses existing glass. I tried drilling mine for an overflow and cracked it (rookie mistake of trying to drill too fast), but I ended up building an acrylic box of roughly the same dimensions. It worked rather well, but does require pumping water up into the tank and working out that overflow (including an emergency backup since macro algae is a pro at clogging drains you think are protected).
2) Add a baffle (new wall) into the skimmer area just after the filter socks (if you are running them; when I ran my fuge there I removed the socks more often than not). There are kits for this you can find, or else you can just silicone in something yourself (either glass or acrylic sheets). Since this isn't a water-tight area, the seal doesn't need to be perfect, so the pressure is off. This is a great option as it doesn't require any special plumbing or pumping, but does reduce the available space in the main sump chamber.
3) If you remove the ATO tank to somewhere else, you can actually use the first return chamber as a decent fuge. You'll get lots of direct flow and it's the first chamber, so the macros will have first pick of the nutrients. The socks are downstream as well, so all the little bits and such get caught. This does reduce (practically eliminates) any pod population getting back into the tank from the fuge (generally seen as a benefit of the fuge), so it still may be a good idea to run without the socks (or leave out the one closest to the door to capture some of the detritus, but not all of it.
4) An external fuge is always an option for any tank. There's enough of an opening in the stand to run plumbing in and out; just make sure the fuge water line is higher than the sump edge and you shouldn't have any drainage problems. Of course, this likely places the fuge outside the stand, which can be less desirable for some. Still; a nice mangrove tank or the like can be attractive...
Using a UV is a matter or preference - it's not a required piece of equipment by any means. If you do go with a UV, be sure that it's properly sized for your goals and that you can tune the flow to it (which will be much slower than your return line will generally run).
I found the M2 to be underpowered for the 525XL, honestly. It ran at basically 100%, which produced a lot of heat and resulted in a noticeable drop-off when it started to gum up - which resulted in the tank getting loud as the return flow got lower than the plumbing was set to handle. When I upgraded to an L2 I was able to run it at a lower speed, which produced less heat. And if the tank was making a lot of noise due to a flow drop (and I didn't have time to clean it at that moment), I could kick up the pump a bit to compensate. It didn't always work perfectly, but it did help some. I also had enough extra juice to add a manifold and few extra bits (carbon/gfo chamber, chiller, etc.).
Again; welcome to R2R - please keep asking those questions!
