Moving Too Fast

It is very necessary to go slow when starting up any aquarium system.
Too often impatient hobbyists who have made the mistake of heavily stocking a tank within the first few weeks of the initial setup.  No aquarium system is considered “seasoned” until it has been operating for at least a few months.
Many systems don’t achieve maximum condition for upwards of a year or more.  That is when they would be considered to be capable of the greatest “holding capacity”
If you "overload" a tank with too much livestock or live rock all at once you may be asking for “new tank syndrome” whereby you have triggered an ammonia spike and will likely kill all your fish.

Improper medications

You MUST first know what you are treating!  Some diseases look very much alike.  Do your homework.  Research.  Read.
 Most importantly, medication should be done in a separate treatment tank, as most medications will have an adverse affect all the “good” bacteria that you have been waiting propagate and become established in the filter system.

Inadequate Filtration

You can not “over filter” your tank!  Plan according to your expected “bio-load”.
Err on the side of caution when choosing equipment.

Livestock Incompatibility

Think of what is happening on the reef in the open sea.  It is a “fish eat fish” world. Now, overcrowd your “miniature reef” and see what happens.
Even known compatible species can become agitated with one another and a fight will ensue.  Or maybe one inhabitant just decides to make another his dinner.  It happens!  So do your homework and never overcrowd!
Stress related disease can occur when one or more fish are simply uncomfortable with their tankmates.  Do your best to learn which fish are happiest together and then hope for the best.

Purchasing Fish in Poor Health

Always try to observe a fi