Angelfish and Catfish
Over the years, my experience with Pterophyllum has shown them to be very efficient and determined predators of other cichlid fry.
Q. I recently purchased three altum angelfish, and they have been eating and doing very well in a 265-gallon planted community aquarium. Recently, I noticed one of them with its mouth stuck wide open. I then discovered that it had tried to swallow one of my Otocinclus catfish, which was still in its mouth! What can I do to get the catfish out of the angelfish's mouth? I tried to pull it out, but I didn't want to pull too hard because I know the Otocinclus has spines in the fins. Have you ever heard of angelfish eating Otocinclus? I have several regular angelfish, some much bigger than the wild altums, and they never seem to bother any of my Otocinclus or other smaller fish.
A. I'm not surprised that one of your altum angelfish (Pterophyllum altum) has tried to eat one of your Otocinclus catfish. With all the stories one hears about angelfish eating smaller tankmates, such as neon tetras (Paracheirodon innesi), it's to be expected that other small species like Otocinclus would also make the list of angelfish dietary supplements. So, to answer your second question first: Yes, it happens regularly.
Over the years, my experience with Pterophyllum has shown them to be very efficient and determined predators of other cichlid fry. Combined with their reputation for eating neon tetras, one can easily see that Pterophyllum are fish eaters — at least opportunistically — despite the popular notions of refinement and restraint bestowed upon them by their owners. I think your altum was simply trying to do what it would have done in the wild.
Extracting a spiny catfish from the mouth of an angelfish can be difficult, and unfortunately often results in the death of both fish. First, you need to assess the situation in terms of how far the altum got in swallowing the Otocinclus. If the pectoral and dorsal spines have locked in place and prevented the angelfish from getting it down very far, you may be able to gently wiggle the Otocinclus out.
It's likely, however, the fin spines will be stuck firmly in the sides and roof of the angelfish's mouth, so significant movement is simply not possible. If that's the case, then a surgical extraction is needed. A very small pair of scissors can be used to cut off the pectoral and dorsal spines of the Otocinclus so the catfish can be removed. Unfortunately, the procedure will probably result in the death of the catfish.
If you decide to go this route, remember to remove the spines from the altum's mouth before you release it back into the aquarium. The procedure will also be extremely stressful on the altum, so any effort to get the angelfish back in the water and keep the procedure as brief as possible will help the angelfish to recover. There is, of course, the chance that despite your best care the altum will succumb to the stress.
Hindsight — taking at least some advantage from this experience - should cause you to re-think the community of fish you have in the aquarium. I suggest replacing the Otocinclus with a larger species of algae eater to deter your angelfish from attempting to eat such a dangerous meal. There are larger Otocinclus occasionally available from specialty importers; research some of the alternatives and ask your local aquarium store to order some for you.
As your altum grow larger, your current Otocinclus will look increasingly smaller to them, making these little catfish even more tempting as a meal. I'd replace them with larger alternatives before another of your altums bites off more than it can chew!