What are they?
Phages (bacteriophage) are a special group of viruses which consume bacteria. They live naturally in places where bacteria live.
This group of viruses have evolved over millions of years to specialise in hunting down and killing only one species or family of bacteria per virus and unlike antibiotics, bacteria can't develop resistance to them as the viruses themselves evolve to match the bacteria defences.
Scientists have recognised for a long time that antibiotics are slowly not working effectively any more as bacteria learn how to avoid being killed by them. This is becoming a serious problem in human and animal medicine. Already antibiotics are banned from use by the general public in several countries, with more following suit in order to preserve what limited effectiveness antibiotics have left.
Thankfully an alternative to antibiotics has been found in the form of these viruses.
There are many millions of species of these phages and whilst not yet as well researched as antibiotics in the Western world of medicine, this is changing as an alternative to antibiotics will be required in the future.
To date only a few dozen species of phages have been discovered. But already scientists have discovered a phage (called Φ6) that kills the bacteria - Pseudomonas. A well known bacteria that causes fin rot.
It is not unreasonable to believe that others will be discovered given time that could kill other infectious bacteria and eventually be made available on sale at some point to aquarists.