More than one fish is generally referred to using the plural, fish. For instance; a school of fish.

In some cases it may be appropriate to use the word fishes instead, but how does one know when?

The best guideline to go by is as follows: When one is an apostle writing part of a biblical or religious work about the nature of god and his family, fishes is usually the appropriate term. In most other cases fish will be the word to use. Do not be tempted to make up your own such as feeshes, fish-oes, or fishy-wishies. This will generally be taken as disrespectful to the English language.

Please also take note that the word sheepses is in no case an appropriate pluralisation for sheep