With that large of a pH drop, beside adding oyster, shell as RD suggested, I think you should normally be doing more water changes than once per week, they should be larger than 30%, and the filter should be rinsed at least once per week.
I highly doubt that the crushed coral caused your issues with your Asian aros tail. Never heard of such a thing, with coral, or oyster shell.
With that large of a pH drop, beside adding oyster, shell as RD suggested, I think you should normally be doing more water changes than once per week, they should be larger than 30%, and the filter should be rinsed at least once per week.
oh not Oyster shells RD, Crushed corals. I've ready if the crushed corals are old they have to be changed , Not sure why but did not believe it too before until i experienced it myself .Interesting. I have never seen oyster shells cause issues with Asian aros. Having said that I agree with Duane, sounds like you also need more water changes (and possibly filter maintenance) to keep parameters in check.
With a sophisticated laboratory and someone with the right expertise.btw just curious, how do you measure amount of waste a fish produces?