Celestial goldfish are one of the fancy types of goldfish. Although all goldfish are the same species, the different varieties have very different requirements.
Singletail goldfish can grow to over 20 inches in length, and require very large tanks with lots of swimming room. The fancy varieties grow to between 9 and 14 inches long, and require about 30 gallons for the first goldfish and 10 gallons for each additional fish. They do not need to be kept in schools, but they are very social and will enjoy having tankmates.
Celestials are one of the most demanding varieties. They need to be hand-fed a few times a day near the surface, since once the food has sunk to the bottom they cannot find it anymore. They require very dim lighting due to those upturned eyes. It is best to keep celestials alone or with other bubble-eye goldfish. Any other tankmates would out-compete them for food, or pester them and might injure them or pop their eye-bubbles. It is very important to keep them in a tank without decorations - no driftwood, no plastic plants, even silk plants aren't a great idea. Soft live plants might be alright. Anything that could pop their eye-bubbles is a definite no.
You might be able to keep one or two celestials in your 25 gallon, since they are one of the smaller fancy varieties. However, I would recommend only keeping one, and that is only if you can handle their special requirements. You will need a powerful filter, as all goldfish are a bit messy and produce more ammonia than a tropical fish of similar size.
Best of luck.
