Sorry you had a heart in throat event. We all have them. Some thoughts, you do not mention the current size of your fish. However, I read 75 gallon, 100 lbs. rock (actual displacement unknown, level of substrate unknown) and based upon a couple of species of fish I read of possible trouble brewing before it actually arrived. A couple of fish you have ultimately will, or do, need a bigger tank. Oxygen depletion of 200 gallons with two fish will probably be slower than a, for example, 50 gallon with ten fish. If the 200 gallon has the equivalent of 25% water displacement due to rocks and substrate, there is 150 gallons of water. If a 75 gallon has the same equivalent displacement ratio, there is just over 56 gallons of water. All else being equal, such as tank depth versus surface ratio etc., which tank will the occupants fair better over night with the main pumps shut off and no water movement?75g with over 100 pounds of rock, HOB filter, two wave makers, HOB skimmer, but been off for two weeks due to motor dead.
Clown, scopas tang, melanurus wrasse, leopard wrasse, fox face, blue devil, gold head sleeper goby (died), green spotted mandarin, blood red shrimp, huge long tip anemone. Tons of mixed corals; limited SPS (oddly elegance coral looks rough, all others ok).
I stopped following lbs. rock/gallon long ago as I just do not like a tank full of rock. Today I favor highly porous, light weight per displacement volume, rock for aquascaping. When I am happy with the scape, I am done. For me, surface area for bacteria is the name of the game, but even more something that looks terrific! More water, less rock.
While you save for that tank control system. there are a lot of countdown timers out there that are fairly inexpensive, $10 to $40, get a plastic outlet box and outlet and wire up one yourself if needed. I would probably prefer a good ol fashioned mechanical timer.
Here is hoping the new year is event free for you!!!