i always recommend (and do) weekly to biweekly 20% changes. it's just so that you're not drastically changing your params in a very short span of time. for example, i dont bother heating my water before a change, so a large change could bring down my temp really fast. salinity might change too if your not careful with top off. etc etc.
i think there is a more safety in smaller more frequent changes, rather than the larger infrequent changes (which would cause more stress during the change process). while it is on the large-ish side, i dont think 50% is really big... 75% would qualify though. 100% is a definite no no in my books.
during emergencies, large cahnges dont always make the most sense. I agree that sometimes it's a good idea, but certainly not always. especially if you're not really even sure what the problem is. a 30% change during those times, i think is enough most of the time just to give you time to figure out what your problem is.
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