I have a new build (March) and have been sloooooowly adding to it. 75 gal +20 gal sump (85-90 gal total capacity), 2 x Kessil A360N, 50 lb of BRS Fiji Live, 2" sand and a 1" Marine Pure plate buried (my throw back to a plenum in my previous system... BTW it isn't hurting). The the sump is divided with a Vertex skimmer, GFO/Carbon reactor, a small refugium with chaeto with Kessil H80 running at night, and a return area. Critters: 1 x orange stripe gobie, 3 x fire fish, 2 x peppermint shrimp, 4 x nasarius snails, 4 x trochus snails, 1 x fighting conch, 1 x pom pom crab; small colony of zoas, small colony of xenia, small yuma, small open brain. I left it to my non-reefer neighbor with an ATO hooked up to a 20 gal Brute barrel monitored by APEX with a Neptune AFS (totally awesome). Oh, and right before I left I set up an Arlo security cam directed at the tank with cabinet doors open so I could see sump levels, skimmer overflow, and general condition. I left for 24 days in the beginning of July with a stable tank, but nitrates a little higher (~<10ppm) than I had hoped.
I held my breath and returned two days ago, and for the most part I could not be more pleased... The gobie is gone, (not eating before I left), but aside from that and some algae on the glass the tank is phenomenal.
Firefish are happy, and inverts and corals all look great. Nitrates have continued to drop (now 0 ppm and chaeto is 50% bigger), PO4 still 0 ppm. Alkalinity did take a hit (was 8.2, now 6.4 until water change today), but an interesting thing happened: with the cabinet doors open the Ph was up about 0.2 the entire time I was gone! Literally, the days before I left I was right at 8.1 during the day, 8.0 at night. When I left on day X, APEX readings showed Ph ranging 8.2-8.3 when my normal Ph was ~ 8.0-8.1-ish. I can only attribute this to the cabinet doors being open and gas exchange. Now that I'm home, the doors are closed and I'm back to my previous normal Ph of 8.0-8.1 I'm not running a fan currently, but may start just to get the airflow and gas exchange which has got to be the reason for the Ph difference (the only variable).
Anyway, I'm sure I'm tempting Karma with my next departure (I travel a fair amount and non-reefer wife maintains the fort with LFS on speed dial), but added a pair of clowns today, and am looking to add a few mixed corals next week.
Aside: got my kids SCUBA certified on the vacation, so now we can all go see it in the natural state!