Prepare the new water before touching the tank
Fill a clean bucket with water treated with conditioner, and let it stand or adjust it until its temperature matches the tank's as closely as you can judge with a thermometer or the back of your hand. Angelfish come from slow, stable water, and a refill that arrives several degrees off is the commonest cause of a sulky, darkened fish after an otherwise careful change. Set out the siphon, towel, and bucket before you open the lid, because an angelfish tank change should flow in one calm sequence rather than a series of trips past the glass with dripping equipment.
Siphon a quarter out, gently around the fish
Start the siphon and remove roughly a quarter of the water, passing the gravel cleaner lightly over open substrate where waste collects, and steering well clear of plants' roots and any territory a pair is guarding. Angelfish usually hold station and watch, but a fish that wedges itself near the siphon tube should be waited out, not waved away. Keep the intake below the waterline until you reach your target level, and stop before the filter intake or heater is exposed; if the level will drop near equipment, switch that equipment off first and note it, because the restart check at the end is part of the job.
Refill as if pouring for a sleeping fish
Pour the new water in slowly, aimed at the glass, over a small saucer, or through your cupped hand, so it sheets into the tank instead of jetting across it. A hard refill stream shoves tall-bodied angelfish sideways, stirs the substrate you just cleaned, and can flatten delicate plant leaves against the glass. Fill to the original line, watch the fish for a minute as they re-settle, and confirm the filter is flowing and the heater light cycling before the lid goes back on. A pair that was guarding eggs may inspect the territory anxiously for a while; that settles faster when the whole change was unhurried.
Keep the fraction modest and the rhythm weekly
A quarter changed weekly suits an angelfish setup better than a rare, drastic overhaul, because stability is what this species rewards. Bigger changes tempt keepers who missed a week, but doubling the volume doubles the temperature and chemistry swing the fish must absorb, so a missed week is better answered with the normal change now and a second normal change a few days later. If several angelfish show clamped fins, refuse food, or hang in a corner for more than a day after a change, test the water, compare the refill's temperature honestly, and consult an aquatic veterinarian if normal behaviour does not return.
Hand the weekly change over without losing the details
The details that make this change gentle, matched water, a slow pour, the equipment restart, are precisely the ones a substitute keeper cannot guess. Writing them into the duty itself means the fish get the same change from any pair of hands. In KinsLe, Care Teams lets you assign that angelfish water change to whoever is on duty and timestamp its completion, so the tank's steady weekly rhythm survives holidays, late shifts, and helpful relatives alike.