If you're changing 90% of the water daily, then the water in your tank is essentially the same as the water in your geographic water table. I change 100% daily and the water going in is the same as the water going out. (My water is conditioned 24 hours.) As far as the fish are concerned, pH, KH, GH do not change. That's the beauty of large water changes. If you're changing this much water, there are no pH crashes at all. I'd get rid of the crush coral.

Your sponge filter is the most biologically active bacterial reservoir because of the availability of oxygen. Canister filters are typically limited by the amount of oxygen, so bacterial growth is much less robust than open filtration systems. In addition, the amount of surface area is a sponge is orders of magnitude larger than what's available in bioballs. I've converted my entire fishroom to running only sponge filters. They also never accumulate gunk.

Good luck, Willie