The Chinese character for fish (yu) is pronounced the same as the Chinese character for "abundance" or "surplus". The fish symbol is, therefore, frequently associated with other symbols and Chinese characters to symbolize the wish for "more" in the sense of "more wealth, good fortune, and long life. The character for lotus (lian) is pronounced the same as another character which means "in succession or one after another". Thus when lotus and fishes are together, it means "more and more (wealth/ fortune) year after year.
Some customers questioned about the accuracy of this explanation as they read online, especially from some tattoo related website, that when fish and lotus are together, it is more important to emphasize the type of fish (which would be Japanese koi fish). According to those websites, koi fishes have such deep meaning in Japan as its spirit to swim against the current of Japan's Yellow River to arrive in the dragon gate. What is called koi fish in Japan is in fact the carp fish of China. The story of the carp jumping over the dragon gate is a valid story, except it is an ancient Chinese story and the Yellow River is the 2nd largest river in China. However, when the fish and lotus are together, the type of fishes doesn't really matter. In a lot of Chinese paintings or other art forms, you will see other types of fishes such as golden fishes.