The answer is yes, and no…
All the fish commonly available at fish shops that eat algae (e.g. Chinese algae-eaters, plecostomus, kissing gourami) require tropical water conditions (i.e. ~24 degrees Celsius) to survive. Koi and goldfish, on the other hand, can tolerate water temperatures of close to freezing, to 30+ degrees Celsius. Their wide temperature tolerance is what makes them ideal for unheated outdoor ponds in most parts of the world.
If you live in a tropical country (i.e. near the equator), then you can happily house many different fishes in your pond, including algae-eaters. But if you live where there is significant seasonal temperature differences (spring, summer, autumn/fall, and winter), and intend to keep tropical fishes, you’ll need to maintain water temperature at ~24 degrees Celsius with a heater.
How many algae-eating fish can I stock in the pond?
The number of algae eaters will largely depend on how many fish are already in the pond, and your filtration capacity, the stage of maturity of the biofilter (familiarise yourself with "new tank syndrome"), and the amount of algae available. The latter would be one of the most important factor because these fish are grazers, and tend to eat slowly. If they are housed with fish that have a ravenous appetite, then they can only rely on the natural algae growth for their food. My suggestion would be to stock algae-eaters at a rate of 1 per month, until you reach a happy balance between the number of fish and available algae.
Dr Richmond Loh
DipProjMgt, BSc, BVMS, MPhil (Pathology), MANZCVS (Aquatics & Pathobiology), CertAqV, CMAVA, NATA Signatory.
THE FISH VET, Perth, Western Australia, AUSTRALIA.
Mobile Aquatic Veterinary Medical & Diagnostic Services.
Ph: +61 (0)421 822 383
President WAVMA 2014