Tropical fish at those cooler temperatures become lethargic and prone to ich. The compromise zone at 23-24C is suboptimal for both. Singapores ambient 28-32C means you need a chiller for any serious goldfish keeping and only then can you stock any tank mates at all.
Goldfish prefer cooler water, which limits their compatibility with most tropical fish. Many tropical species cant tolerate the lower temperatures goldfish thrive in, and even those that can may not be ideal companions.Cool water community aquarium fish infographic.
Once you have an established tank and all of the fish are healthy and relaxed, moving the tank or the fish can cause great upheaval. It can even lead to injuries and stress-related sicknesses among your fish. So, how do you safely transport goldfish?

This particular example perfectly highlights why Small Fish Safe With Goldfish In A Cool Water Community Tank is so captivating.
3. The Unheated Aquarium. Cold water fish are cool because they can live in lower temperatures, whether its in a chilly office building, basement, or living room tank with no aquarium heater.
Goldfish like cooler water and dont need a heater. Tropical fish need warm water to live comfortably. These different needs mean they cant live in the same tank.

Keeping a tropical fish in cool water will stress their immune system, making them prone to illness. Keeping a goldfish in water thats too warm will speed up their metabolism, shorten their lifespan, and reduce oxygen levels in the water.