1. Always use a new or clean bucket when going through the acclimation or water changing process. A bucket that has been used to hold bleach or other chemicals can be very harmful for the fish in your aquarium.
2. When adding new species to your aquarium, you have to ease them in. Adding too many fish too quickly can cause the ammonia and chemical levels to raise quickly. Adding fish slowly gives your filter time to catch up.
3. Try to provide an environment that is as stable as possible, fluctuations in the aquarium can have a negative impact on the aquarium. Try to keep the temperature and PH levels fairly balanced in your aquarium.