I change out about 100l at time.
-First is I'll close the valve off the return pump so don't drain the display, then turn off the return pump
-siphon ~60l from the display focusing to get any settled detritus
-open valve and turn return pump back on to get the corals back under water, then valve closed and pump off
-siphon the remaining 40l from the sump and fuge
-transfer the new 100l to the sump turning on the return pump after added ~50l
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