Goldfish and angelfish need different water from the start
Goldfish and angelfish should not share a tank, and the temperature gap alone is enough to settle the question. Goldfish are cold-water fish that stay healthiest in unheated water well below what tropical fish require, and they slow down and feed poorly if kept permanently warm. Angelfish are warm-water river fish that become sluggish, lose appetite, and fall prone to illness when their water sits at goldfish temperatures for long. There is no comfortable middle setting: warm enough for the angelfish means a goldfish living its whole life too hot, and cool enough for the goldfish means an angelfish that never truly thrives. Any pairing that starts by asking one fish to endure the wrong temperature every day is not a pairing to build.
Waste, size, and appetite widen the goldfish gap
Waste and growth make a goldfish and angelfish tank harder to keep safe even if temperature were ignored. Goldfish eat greedily, digest inefficiently, and produce far more waste for their size than most tropical fish, so their water needs large volume and strong filtration to stay clean. An angelfish in that environment faces water quality swings it is poorly suited to, while also competing with a faster, bolder feeder that will take food from right in front of it. Goldfish also keep growing for years and investigate everything with their mouths, including the long, trailing fins of an angelfish, which can end up torn without any real aggression behind it. Add the goldfish's habit of uprooting plants and stirring the substrate, and the calm, structured space an angelfish needs steadily disappears.
Why a short stay together is still a bad idea
A temporary goldfish and angelfish arrangement carries the same problems in miniature, so it is not a safe halfway step. Even a few weeks at a compromise temperature stresses the angelfish's immune system and speeds the goldfish's metabolism and waste output at the same time, making the shared water dirtier exactly when both fish are least able to cope. Moving fish between tanks also spreads any parasites or infections from one setup to the other, which can turn one housing problem into two sick tanks. If an emergency forces a brief share, use a divider, test the water daily, and treat it as days, not a season, while a proper second tank is prepared and cycled ready to receive one of the species.
What to do if these fish are already in one tank
Separating goldfish and angelfish that already share a tank should be planned around the new tank, not rushed around the argument. Set up and cycle the second aquarium first, matching its temperature to the species moving into it, then move the angelfish into warm, calm, planted water or the goldfish into a spacious unheated setup, depending on which tank already suits which fish better. Watch the separated fish closely for the first fortnight: an angelfish that was chilled may show clamped fins and poor appetite as it recovers, while a goldfish kept too warm may be unusually lethargic. If either fish shows laboured breathing, sores, or stops eating entirely, seek advice promptly from an aquatic veterinarian, because temperature-stressed fish can decline in ways that are not obvious from behaviour alone.
Separate tanks, clearly labelled for everyone who helps
Two tanks with opposite rules invite exactly the mistakes family helpers make, such as adding warm water to the goldfish tank during a change or feeding tropical portions to fish that need far more. Writing each tank's temperature, feeding, and stocking on its own profile stops those mix-ups before they happen, and KinsLe Family Sharing lets the household keep the goldfish setup and the angelfish setup as distinct shared records that anyone helping can check in seconds.