There was a piece on Radio 4's Today programme this morning about octopuses, and how intelligent they are (available to listen to here for another week - go to 1:24:45). The octopus expert from Brighton's Sea Life Centre, Kerry Perkins, said that the Centre was hoping to persuade people not to eat octopus.
I felt that she would have added extra weight to her argument if she had not used 'octopus' as the plural form. She said, for instance, "We don't actually locally have that many octopus", and described an individual creature as "... one of the most challenging octopus I've had to deal with", and "... one of our smaller octopus".
Octopus used as a plural form is grammatically correct; that is not the point. Many creatures have what is termed grammatically a zero plural, as well as a regular plural form, ending in -s, or -es, but when the animal is talked about as a group en masse, which it often is when considered as prey - something to be hunted, and/or eaten - there is a collective plural form, which is the same as the singular. I would therefore say "I eat rabbit" but "I have three pet rabbits". Although passing out of the language, largely because hunting animals is not such a widespread pursuit these days and thus a less frequent topic of conversation, examples can be found in literature of sentences such as "He bagged a dozen tiger" or "he shot duck".
Animals that have a collective plural include fish, buffalo, deer, bison, elk, and grouse. Even insects can belong to this group. So, you might hear a gardener complaining about "these annoying greenfly". There is no hunting or eating going on here, of course, but the singular form is used when the speaker regards the creatures as a species, not as a group of individual flies.
The use of octopus as the plural form, therefore, is fine when referring to octopus as food, but since the whole point of Sea Life's campaign is to prevent people eating octopus, it might be wise to say octopuses when referring to the inhabitants of their tanks.
Some people think that the plural of octopus should be octopi and, indeed, this variant is given in the OED, albeit as the second option after octopuses. Octopi is not, strictly speaking, correct, but exists because of confusion, or ignorance, of the etymology of the word. Latin words ending -us, eg fungus, radius, nucleus, often form their plurals in -i (not always however - virus is one that doesn't), but octopus is originally from Greek, not Latin. The ancient Greek plural of octopus was octopodes, which is also given as a variant in the OED, but is rare these days, and would be considered pretentious or pedantic.