More fish die from a skipped setup step than from almost anything else — and it's a step that isn't obvious unless someone explains it first.
The nitrogen cycle
Fish waste, uneaten food, and decaying plant matter break down into ammonia, which is toxic to fish even at low concentrations. In an established tank, colonies of beneficial bacteria living mainly in the filter convert that ammonia into nitrite (also toxic), and a second type of bacteria then converts nitrite into nitrate, which is far less harmful and can be managed with regular partial water changes.
The problem: those bacteria colonies don't exist yet in a brand-new tank — they take roughly four to six weeks to establish in meaningful numbers. Adding a full stock of fish to a brand-new tank before this cycle is established is one of the most common reasons new fish die within the first weeks of a new aquarium, a situation experienced fishkeepers call "new tank syndrome." Cycling a tank before adding fish — or adding only a few hardy fish very gradually while monitoring ammonia and nitrite levels — gives those bacteria colonies time to establish before they're needed at full capacity.
Bigger tanks are actually more forgiving, not harder
Counterintuitively, larger tanks are generally easier to keep stable than small ones, not harder. A larger volume of water dilutes waste products and buffers against sudden swings in temperature and water chemistry more effectively than a small volume does — a small tank can go from healthy to dangerous much faster than a large one when something goes wrong. Starting with a larger first tank than you think you need is often a genuinely good beginner strategy, not an unnecessary expense.
Filtration needs to match bio-load, not just tank size
A filter's rating is usually based on tank volume, but the real driver of how much filtration is needed is bio-load — how much waste the fish actually produce, which depends on the number, size, and species of fish, not just the water volume they're in. A heavily stocked tank of larger, messier fish needs more filtration capacity than the same size tank with a few small, low-waste fish.
The one thing people forget
Test the water regularly with an actual test kit, especially during the first couple of months, rather than judging water quality by how clear it looks. Clear water can still have dangerous ammonia or nitrite levels — visual clarity and chemical safety are not the same thing, and by the time a problem is visible (cloudy water, sick or dying fish), it's often already been building for a while.