Once the filter/system is stabilized and you have proper buffering the PH will only change under the following:
1. Major water change (keep it to a max of 1/3 the water per day) 2. Fish croaked 3. Filter crash due to water flow stopping. 4. Minor amounts may be due to dissolved CO2, but in this fish density I am assuming you will be circulating a lot of water, no plants, and bubbling air. The CO2 wont last long.
I only test PH when I am about to change water to make sure my change water is within 0.5PH of the aquarium. I only change water about once a month since my filter is over-sized. If PH doesn't match, I have an opportunity to adjust before the change. I could alter the aquarium, but it is better to alter the change water unless the aquarium is not near where you want it. So far for the past two years, my aquarium water and tap water have been exactly the same without need for adjustment.
That last PH problem is a stinker. If power goes out, it is time to service the filter getting rid of as much organics as possible, but DO NOT STERILIZE. This is critical because without water flow, your filter will become anaerobic very quickly and kill beneficial bacteria. Some people make the mistake of using chlorinated tap water to clean up, which can kill a lot of the bacteria. The filter bacteria colony is more important than the fish when it comes to an easy to maintain system. It is also why if you ever need to medicate, it is best to take the filter out of the system for a while, just relying on water changes to see the fish through.
BTW it can take a few weeks for a filter to stabilize, and in the case of a heavy load, you start a filter this way: 1. Set up aquarium with new filter, etc. salt, buffering, and so on.Get it running. 2. See if you can grab a little used filter media from someone else to seed the filter. You may be able to simply swish you filter media around in some pond water to pick up some pond flora. Stuff it somewhere into your filter. 3. Let settle for a couple of days with everything running. This is the time to monitor PH, Nitrates and Nitrites if any are present. It is this time period where there will be wild swings in almost all parameters, and where most people fail at starting a new aquarium since they don't want to wait for things to settle. 4. Start adding fish, just a few hardy "break-in" fish. These guys will feed the filter. Feed them as much as they are willing to eat. You want them to "do their business" Wait a couple more days. 5. Check your PH, and nitrogen compounds for a little while longer until they stay the same and your "toughies" seem to be happy. 6. Start adding your desired fish. Preferable a few at a time to let the filter catch up.
I am unsure how to accelerate this process, maybe have the filter running water from an already established aquarium in tandem with the existing filter, and use some existing water as well in the new setup, but then you tend to run a higher risk of disease transferal.
As far as power outages, use a computer UPS to keep the filter and any aeration systems running. Don't bother with a heater from the UPS, they take too much power. The problem with congested aquariums is that without power, you are done on only a day or two. A UPS may give you enough time to get through to when the power is back on. An aquarium that size with a lid will stay fairly warm for quite a long time, and even more if it is insulated.
You probably want an "emergency heater" just to keep the temperature from going below 39 degrees. Any old 100W heater will do if you can set it that low. You may need 2 of them, but to raise the temp only a few degrees wont take much. The circulation pump may be enough already as that will add heat to the system, especially if it is a submerged pump.
