I've kept a lot of self-collected wild fish over the years, although I can no longer do so due to tightening legal restrictions. This is in addition to many store-bought fish, both wild-caught and domestically bred. When I do a water change...which is often...I rarely bother with less than 80% and it's usually closer to 95% or so. One of my old mentors used to refer to these as "fin level" water changes, meaning that the fish are down on the bottom, often tipping onto their sides, before the water draining stops and the filling begins. The fish always seem to love it, usually looking their best as the new water fills. I don't think I've ever lost a fish due to a high-volume water change; perhaps someone can illustrate an example where this plan wouldn't work, but I've never seen it.
Doing such a change on my 360-gallon takes about 1.5 hours, which is spent in my recliner just keeping an eye on things. The only work involved is opening and closing valves.
I like to do one such high-percentage change at least weekly. I think frequency is important. Don't change the water because it has gotten dirty; change it before it has a chance to become dirty. In a moderately-stocked tank, a weekly large change means that the water going out hasn't changed or degraded significantly, so the fresh water coming in doesn't shock the fish in any way. I'm on a well, but even if you are paying for municipal water, it's still the cheapest part of aquarium maintenance; far cheaper than the electricity to run/heat/light the tanks. Don't skimp on it.
I subscribe to the idea put forth by
duanes
regarding keeping fish that are suited to the water I have; I don't want to fight nature. If I had to manufacture water like so many folks do...reverse-osmosing, re-mineralizing, de-chlorinating, fiddling with matching pH/hardness...I'd get out of fishkeeping.
I like your idea of storing the water in vats or bins; although I have installed an on-demand system with a temperature-controlling mixer valve, I usually take the new water out of one of my storage tanks which are at the same ambient air temperature as the fishtanks. You can always add a heater to the storage tank if your fishtanks are heated above room temperature. A storage tank is also the perfect place to de-chlorinate if you are using municipal water. Typically, I drain down the fishtanks, refill them from the storage tanks, and then refill the storage tanks immediately afterwards in preparation for the next change. If you put in a bit of effort to find and mark the fishtank at the level that will allow the storage tank to drain completely into it for the refill, and mark the storage tank at the appropriate level, you don't even need to be present while the new water refills the aquarium...although I like to be present.
My system is not "automatic"
per se, because I have to be there to open and close the appropriate valves to drain, fill, etc. but I don't need to carry hoses or buckets to do so. Water drains from the tanks by means of a pump, a series of semi-permanent hoses, and a valve-controlled manifold which allows me to control which tank is drained, the water being pumped to the outdoors. Then new water flows by gravity from the storage tanks to the fishtanks; the storage tanks are in the crawlspace several feet above the basement floor.
The climate in my locale requires me to pump the old water a considerable distance from the house during winter to prevent the formation of a skating rink. During the warm months, the old water is pumped to wherever in the garden it might be useful, or out into the pasture, or even to top up my backyard pond or stocktanks in dry weather. If you live in a warmer climate, it simplifies the disposal of the old water considerably. If your home is served by a municipal sanitary sewer system, it's even easier.
Yes, setting up the system in the first place takes a bit of time and effort, but the time and effort saved in the long run far, far outweighs that minor, one-time inconvenience. A water change spent quietly observing your fish, reading, listening to music, enjoying a beer, etc. is an absolute joy rather than a chore.
