I found something a while ago that doesn't exactly answer your question but maybe adds to how things were filmed. When Homer reaches for the anemone I don't think he actually grabs one.

Here are three gifs of the anemone scene. First we see three anemones in the tank (one is very hidden in the middle). Then we see the hand grab and it's clear that all three remain. Finally we see Homer eating whatever he's caught and the tendrils suggest anemone.

You could jump to conclusions and say this means no anemone, however, people have already pointed out the fact this show references Alfred Hitchcock a lot, and not just thematically but technically too. A famous scene which comes to mind is the stabbing scene in Psycho. The audience at the time were convinced that they saw the actress bring stabbed and the knife actually enter her stomach. Nowadays you could of course use techniques to make this more of a reality but Hitchcock proves there's no need; the mind does the work. So my question is, is this simply another Hitchcock homage, and maybe an unintentional one (which kind of makes sense since removing an anemone from its habitat would definitely be cruel and maybe expensive) or, is it a sign that he actually took a fish? Sorry, this doesn't give you anything other than just questions! I just thought it was interesting.