I have a best hits, and with less than a year under my belt:
* Believed pH was a measure of alkalinity, so dumped about cup of baking soda into small DT "to increase pH" - LESSON: Increasing sodium bicarbonate DOES NOT EQUAL an increase in pH (also, it's incredible difficult to maintain an elevated pH, so probably not terribly worth the effort and expense)
* Believed that H2O2 was a cure-all for coral issues, so ended up bleaching my first batch of corals after soaking them in the stuff - LESSON: DO NOT dip your corals in H2O2 UNLESS you know it will help; proper quarantining of everything that goes in your DT will take care of any pests that H2O2 would
* Did not have a quarantine system set up, so lost about $300 worth of fish within 4 weeks of introducing some newbies into the DT - LESSON: SPEND THE MONEY AND THE TIME TO QUARENTINE
EVERYTHING THAT GOES IN YOUR DT
* Didn't properly cycle QT and hit a nitrite spike that wiped out a recovering fish therein (following the aforementioned plague) - LESSON: Seachem Prime is your best friend if you even THINK your biofilter might not be set up or is compromised in your QT/HT (dose daily)
* After setting up a HT system following the aforementioned plague, did not know it was necessary to thoroughly oxygenate medicated water, so part of the fish loss referenced above was due to suffocation - LESSON: Spend $40 on a good pump, tubing, and air stones and aerate that sucker
* Accepted the gift of a free 75g aquarium, which led to most of the aforementioned problems - LESSON: Never fool yourself into thinking you know what you're getting into with this hobby
One final lesson: