Use a chlorine remover when doing water changes to save your nitrogen cycle, especially in those super rare times you're forced to do more than 30% water changes, which brings me to #2...
Hide your
fish food when friends with young kids come over. It seems like an addiction for them to randomly pour a whole bottle in your tank. I've had this happen 5 times with 3 different kids so far.
Never change all filters at once. Set it up so you alternate. This will prevent
ammonia spikes.
Check fish temperaments before purchase. I now have a 90 gallon intentionally overcrowded
African Cichlid tank because my wife thought 1 looked nice at the LFS for our 20 gallon tank, lol.
Buy
bristlenose plecos, not common plecos; common plecos are meant for lakes and ponds, and it's cruel to keep them in a tank under 120 gallons because they won't grow to full size, and their lifespan will be cut severely short.
If you're absolutely forced to
euthanize a fish (incurable
disease, birth defect, etc) put them in a clear jar and place them in the freezer for an hour. It's the most humane way to do it, which leads me to my next one...
Don't buy fish that have been genetically altered. This is a stupid and cruel practice. Those GloFosh are made by taking regular healthy fish species and genetically modifying them with jellyfish DNA in order to make them glow under blacklight, which is horrible for fish anyway and can blind them after a while. Pygmy species are also usually genetically modified, such as
dwarf gourami species. What happens is that their limited gene pool results in massive inbreeding and loads of genetic diseases and defects. Every fish I've had to put down was genetically altered, and I've never had a single dwarf gouriami last longer than 6 months.
Water agitation is massively important and the only purpose of aerators. The surface of the water is where your oxygen exchange happens, and most fish are used to a pretty strong current. A calm water surface is a gateway to disaster.
Lastly, buy plants from sources that are reputable, and quarantine them separately from every other fish and QT tank you have. Plants don't need much for a short QT, but once you get snails in your main tank, you will never get rid of them. And snails and filters don't mix well with the type of pest snails I'm talking about. This came about because of some
duck weed I bought from a source that promised no snails (
cichlids love duck weed). Anyway, now I have to pull my filters and remove the snail infestation every couple months.