Livebearers as guppies (Poecilia reticulata) and mosquitofish (Gambusia affinis) can be very useful in the tank systems of public aquariums and other fish-holding facilities. They are fairly easy to raise in moderate to large numbers in a facility. When raised in a facility, as opposed to being collected in the wild, they do not normally carry various internal parasites that might be passed on to your display fish when they are eaten unless they are raised with recently caught wild snails or other possible intermediate hosts.
Since both guppies and mosquitofish, though usually found in freshwater habitats, can also survive in salt water, both can be used for various tests in fresh and salt water tanks. Guppies prefer hard water and can survive in salinities up to 1.5 times that of sea water (Wikipedia), or in excess of 52 ppt. Mosquitofish can tolerate up to about 40 ppt salinity. Acclimate fish to the salinity of the test tank before introducing them.
When used in various tanks for tests, they can then be observed or sacrificed to verify/confirm that various toxins or diseases/parasites are present in the tanks. By using the smaller livebearer fish you are not sacrificing your display animals to possibly find out what might be going on in a tank.
Test water quality
After a new tank is set up or a re-worked tank is ready for display, livebearers can be introduced to give you a “survivability” test. If the livebearers survive at least two days in the tank without signs of stress or disease then the water quality, toxin level, and bacterial presence in the tank can be considered safe enough to support your more expensive display organisms. You can leave the test fish in the tank (inside a floating net chamber if you want to sample them later) so you can test the tank for a longer period of time. Or you can leave them free in the tank so that the new display occupants can chase some live food and thus get food and exercise.
Test water toxicity
Livebearers can be used to verify a new tank (or re-worked tank) is ready for display organisms and has no strong toxins (from chlorine, fiberglass, paint, aquascaping, etc.) in it. Introduce several fish to the tank and observe over the next few days. If the fish do not show signs of stress, or die, you should be able to add your display organisms. You can leave the livebearers in the tank for exercise and snacks for your display fish.
However, low (non-lethal) concentrations of toxins may still be in the tank. Low levels of toxins may not cause a noticeable stress response in the fish, and may, or may not, cause problems over a long period of time. You can place several fish into tank floating net chambers, sample them over an extended period of time, and then analyze the fish for any increase in toxins (above the concentrations that were already in the fish prior to exposure in the test tank).
Test for presence of some parasites/diseases
Acclimate livebearers and introduce them to tanks (inside floating net chambers) with suspected parasites or diseases that can also infect and affect them. You want to sacrifice them rather than expensive or hard-to-collect/obtain display fish.
One example: I was the first to find that guppies held in salt water could be infected with Amyloodinium ocellatum in l984 (unpublished), and had previously found (Lawler, l980) that another freshwater fish held in salt water could be infected with the dinoflagellate. I passed my findings on to Ed Noga, who then developed guppy tissue cultures that he could use in studies on A. ocellatum in the laboratory (Noga and Bower, 1987). He kindly acknowledged my help.
I also used my findings to employ guppies in display tanks of the Scott Aquarium in Biloxi, Mississippi, to check for the presence of Amyloodinium. I could then move my limited number of diatom filters to a tank to control any Amyloodinium (Lawler, 2007) that might have survived treatment procedures used on the display fish prior to display, or Amyloodinium that somehow got introduced to the tank.
If you suspect Amyloodinium ocellatum is in a tank, put some livebearers inside a floating net chamber in the tank after they have been acclimated to the tank salinity. Test fish should be separated from display organisms in the tank so they are not eaten, or killed. At daily intervals, remove a fish and examine it with a microscope for the presence of Amyloodinium on the skin, fins, and gills. If Amyloodinium is found, put a diatom filter on the tank to filter out infective dinospores.
Livebearers can also be used to verify the presence of other parasites that can attack a wide range of hosts (having low host-specificity), like Ich, and also various bacteria that may cause disease problems in tanks.
As fish food
Livebearers raised in the lab and thus parasite-free (as opposed to wild-caught fish which may have all sorts of internal parasites that might be passed on to other organisms) can be introduced to display, or holding, tanks as live food for fishes. The live food can be eaten when the fish want to eat, rather than the display fish being on a set feeding schedule determined by humans.
Livebearers can be introduced to various tanks that contain organisms that eat live food so they get some exercise chasing the live food. The chasing is more activity in the tank which may appeal more to your viewing audience.
As tank cleaners
Livebearers can be used in tanks to clean up small scraps of food missed by larger organisms.