Pacus For An Outdoor Pond?

Large aquariums are expensive, so is a pond a good choice for a too-large fish?

Black Pacu (Colossoma macropomum). Via Prayitno/Flickr


My husband and I have a dilemma on our hands and we really need help. We have two pacus (Colossoma macropomum), approximately 4 years old, living in a 125-gallon aquarium. I’m sure you can appreciate how inappropriate and utterly inadequate this setup is considering the size of these fish and their growth rates. Of course, we now know that our purchase of pacus was incredibly stupid, but the store clerk assured us that the fish would grow to the size of their environment and no more.

The folks we have talked to locally all admit to turning their problem pacus loose in a large local river (which accounts for those summer reports of piranhas being found in our area) or having a big fish fry. Both of these seem irresponsible to us, and we’d like to come up with something better.

We have priced custom acrylic aquariums and found the price to be staggering. So we come to our dilemma: What do you with these gentle giants?

We think a backyard pond is good solution because the price of setting one up is more within our means and we can increase the size as needed. My husband is concerned with several things. Just how big can these fish get? We have heard that they can run 60 pounds or more. How big would our backyard pond have to be? What about heating? Our climate is fairly harsh in winter.

Would it be easier to try to relocate these fish to a more natural environment? If so, where? How long can these hardy guys live, anyway? We’re in our 30s — is it possible that they could attain that age, too?

No one in our area is willing or able to give us any ideas, and most people think we are just plain silly to be worrying about two big old fish. What do you think?


Colossoma macropomum is commonly referred to as the black-finned pacu. It is a native of the Amazon and a vegetarian. As you discovered, it can grow to be a big fish — over a foot in length if properly cared for. I think you might expect them to reach 10 pounds or so under confined conditions. Although your aquarium can hold the pair, as you suspect, it is far from optimum.

Pacus evolved in Amazon wetlands where tree roots, emergent and submergent plants, and woody material provide an extensive network of protection. With this in mind you could greatly increase the “habitability” of the 125-gallon aquarium by incorporating vegetative and woody structures that simulate the complex environment of a tropical river — leaving some large open areas for swimming.

A move to an outdoor pond — heavily vegetated with submerged aquatic plants — would be a superior solution. Pacus are used to water temperatures in excess of 70 degrees Fahrenheit, and would certainly flourish in a nice 2,000-gallon backyard pond during the summer, but successful overwintering is unlikely at best. Trying to keep the water in the mid- to upper 60s would be very expensive.

Setting the animals loose is just not an option. As your letter implied, releasing non-native species is an ecologically irresponsible thing to do. Most tropical species won’t survive our winters in most parts of the country, but even a brief existence in the wild is sufficient to transmit devastating diseases and parasites into local populations of native fish. Exotic cast-offs that do survive usually decimate native fish in by predation or, in the case of your pacus, out-competing native fish for vegetation.

In my view, aquarists who release fish into their local streams or ponds do far more damage to wetlands systems than developers and polluting industries. It seems to me that it is a fundamental responsibility of any aquarist or pondkeeper who tires of a pet, or finds himself unable to properly care for the animal, to find it a proper home. This means searching out other hobbyists or a commercial source willing to take the animal — for free if need be. Call some aquarium societies in your region. See if some pet store or commercial facility might want the pacus for a lobby display aquarium.

Article Categories:
Fish · Ponds and Koi