Since tropical fish prefer warmer conditions, it can be hard to find an attractive tankmate for your cold water goldfish. Apple snails are often recommended because they can adapt to most conditions, but even then you have to be careful your goldfish doesn't mistake these smaller creatures for food.
About the Goldfish
As goldfish require a steady temperature of between 65 and 75 degrees, they are rarely compatible with tropical fish. The fish that they can live with are cold-water fish, such as carp, as well as subtropical fish such as a loach, which is able to adapt to most conditions.
Goldfish, no matter the variety, tend to produce a good deal of ammonia, which can ultimately make the tank water high in nitrates. These are potentially toxic and you must make regular partial changes to their water.
About the Apple Snail
Their attractive appearance and relatively big size (2 to 6 inches) have made the apple snail a popular member of many aquariums. In the wild the common freshwater snails inhabit a wide range of ecosystems from swamps and ponds to lakes and rivers. The majority of the species prefers lentic water and only a few species, such as the spike top apple snail, have adapted to conditions provided in captivity.
Keeping Apple Snails and Goldfish Together
Though apple snails are commonly recommended as the perfect tankmate for a goldfish, the website Aqua Puppies warns that a goldfish can mistake an apple snail for food. In general, however, the apple snail, with his hard, occasionally spiky shell, is much too difficult for a goldfish to digest. Your goldfish will most likely only harass an apple snail, if he has not been raised with him in the tank. So from this perspective, it is best to introduce an apple snail to the tank when your goldfish is still very young.
Recommended Fish to Keep with Goldfish
With the ability to adapt to a new environment, apple snails can be kept with most fish. This is not the case with goldfish, which not only require relatively cool water, but can gobble up food before most fish have the chance to move. However, the following types of fish make suitable tankmates: roaches, rubber nose plecos, white cloud mountain minnows, zebra danios and rosy barbs.
Of these, the zebra danio is perhaps the best suited. He is fast enough to both avoid those goldfish who mistake him for a snack and to grab his share of the food.