Why are my fish disappearing?

One of our patrons was concerned about their home aquarium. They had maintained it for many years, but their older fish were starting to die off. They had started to add new fish, but a few of those fish had disappeared. Was one of the older fish eating them? Was something wrong with the environment?

Assuming that the tank was tightly covered, the water was clean, pH balanced, free of harmful chemicals, and an appropriate temperature, and the fish were not being under- or over-fed, we checked Mary Bailey and Peter Burgess’s Tropical Fishlopaedia, Maddy and Mic Hargrove’s Freshwater Aquariums for Dummies, Dick Mills’s Aquarium Fish, Stuart Thraves’s Setting Up a Tropical Aquarium Week-by-Week, John Davies’s Complete Encyclopedia of the Freshwater Aquarium, and www.aquariumcarebasics.com for more information.

Our patron’s aquarium included a Buenos Aires tetra, a rainbow shark, and a rosy barb. They had been attempting to add an otocinclus catfish and a few varieties of platys and tetras.

According to Bailey and Burgess, an omnivorous fish who has never bothered its old tankmates may go after small new fish. If it came into the aquarium at the same time as the old fish, or if it was too small to eat them when they were added, the omnivore seems to recognize them as companions and not food. It’s used to food coming from above – which is also where new fish are introduced. Platys, tetras, and barbs are omnivores, and Thraves mentions that it’s common for larger fish to pick off neon tetras, so that could be a problem. While our patron’s fish are generally known to be peaceful, Buenos Aires tetras can get aggressive as they age, especially the larger ones. Rainbow sharks (which are not actual sharks but are closely related to barbs and danios) are also somewhat aggressive and territorial, especially toward their own kind. Only one should be kept in an aquarium and it should be given plenty of places to hide. (The Buenos Aires tetra and the rosy barb, on the other hand, are schooling fish and are happiest in groups of at least six.)

It’s also possible that the fish were simply unhealthy when our patron purchased them, or that they didn’t take well to the stress of transport, depending on how long they lasted. The otocinclus catfish in particular is known to be delicate and often has trouble acclimating to a new environment.

If our patron is still worried about their fish, Tropical Fishlopaedia and Freshwater Aquariums for Dummies both have information on troubleshooting aquarium problems.