Well no. Fish is singular and plural. However, when you are referring to different species then you use the plural fishes.
Same thing with races of people.
I believe deer can go either way, but it is more common to just use deer as both the singular and plural.
With cacti, it is a similar case to fish and fishes. If you are talking about different breeds, then it's cactuses. However, there is one caveat. Sometimes the plural is the same as the singular.
Generally however, you will only hear cacti as a plural.
Same for fungi and funguses.