I have always considered myself lucky in that mostly my beloved guppies remain healthy and are a pleasure to watch. Unfortunately, that is not always the case and disease can and does happen. Setting up your aquarium correctly, initially, is the key to having healthy fish and healthy water. The aquarium is the environment that will affect your fish and their lives. While this is stating the obvious, so many people will fill the tank with water, throw the fish in and expect great results. Some will be lucky but for many their fish will last a few weeks, look terrible and consequently be found rotting on the bottom of the aquarium, or floating on their backs in some weed.
Factors that will affect the fish’s health are likely to be those of their environment or the water in which they live. Stress in fish lives is a major cause of ill health for them as indeed it is for any of us. For fish, handling is one form of stress that you can control to a certain extent. Understanding the physiological and psychological stresses that may frighten the fish, you will need to empathize with the fish and make any handling, as little as possible, and in the correct manner. Transferring your new fish into their new home gently, allowing the water in their carry bag to reach the same temperature as the tank by resting the floating bag in the tank for at least an hour.
Another stressor is having unsuitable companion species of other fish in the aquarium. Take the time to find out suitable companions for your pets. The constant bullying by another fish will cause stress to the victim and possibly lead to exhaustion and its immunity dropping enough to cause an illness leading to its death.
Given time, fish seem to adapt very well as long as the conditions in which they live are at least the very basic requirements needed for the species you have. You would have to accept that their growth could be limited whether by adapting to the conditions they find themselves in, or by a lowered immunity. Guppies breeding frequently is a given, but this may well be slowed because of getting used to new tank conditions. This may last from a week to several weeks so be aware of the reasons.
Another reason for less than healthy fish are the owners providing too much food , this food lies uneaten on the bottom of the tank and slowly rots. The rotting causes the food to break down, and as it breaks down or rots there are bacteria released, which in turn poison the water. Fish living in bacteria laden water are unlikely to be healthy and happy.
The way to avoid bad things happening is careful planning and attention to the finer details of aquarium care. Manage the watery environment with adequate heating for the type of fish in the tank with a working heater! Ensure purity of the water with judicious water changes while avoiding upsetting the fish .Pay attention to adequate filtering for the tank and the numbers of fish within the tank. Most of all learn from your mistakes and know what works for you and your fish.