What I consider essential. My opinion.

[Sk8r]

A tank large enough for the species I want to keep...right now.

medium-grain sand [fine blows like mad] and a lot of gnarly rock
a wavemaker


A sump large enough to hold:
a return pump that can really move the water
The best heater I can get---cheap and used is dangerous. Plus a digital thermometer for the dt.
An ATO with reservoir: I use kalk in the reservoir, and an Eheim pump is the only one I've yet found that lasts [kalk is abrasive.]
A skimmer that produces foam as thick as egg meringue, so thick it can stand on its own.
Room for spare rock: needs change, corals may need to have a bit glued to them for stability. So a reserve rubble pile is great.

A light that can reach the bottom of my tank: I'm using LEDs. The tank is 30" deep.

a water alarm (against leaks)

spare buckets and towels...because.

A refractometer

Tests and matching supplements for: alkalinity, calcium, magnesium; and kalk; also tests for nitrate and ammonia. Actually I keep Tetra multitest pool strips for the koi pond, and they're fast and a pretty good indicator. You want the real news, get a nicer test kit, but the strips actually aren't bad, if you know what you're looking at.

enough salt on hand to handle my entire water volume, if I had to. Never run out.

qt tank with its own heater, its own ato, and jump protection. Stuff you can sterilize.

filter socks for when I need them. Rarely. {my thing is lps]

more towels.

hose, plumbing parts, bits of filter, old nets---in decades of this hobby, you build up quite a collection.

A jug of white vinegar for soaking pumps and parts.

This is what my collection looks like.

 

[MuShu]

May I add?

-Locations of nearests LFS and which days they are closed. If you don't have an LFS you can run to in a pinch, have fish medicine on hand. I have made more than one after work emergency run to an LFS for meds- mostly for the freshwater tank, but ick and parasites can happen and most fish will be dead if you have to wait for mail order meds.

 

[Sk8r]

Add to that: the internet. It's a bad idea to buy a fish, coral, or invert that you haven't looked up or about which you know nothing. 'What is this?' is one of the scariest questions for a new purchase. Granted acropora corals are really, really hard to id until you look at a whole colony of it, because their branching pattern tells the tale---and nobody can tell you much from one little bit...But for the rest, buying 'because it was there'...is a disaster waiting to happen. Fish stores WILL special order something for you if you miss the one you saw. It's actually good for them, because they know that item already has a buyer. It's how things work. Look it up. Find out how big it gets. Two feet? mmm, not so good. Eats crustaceans. Oops. crustaceans including your 8.00 shrimp? Not so good. Loses color pattern as an adult? Goes dull brown? Maybe not. Look it up! Consult our 'breeds' directory, in Reef Fishes. In Inverts. You want to know it---somebody's almost certainly had one and the info is in one of the forums.