DO I need a...[skimmer, filter, ro/di, reactor, ATO, etc.]

Understand, you can run a very simple kind of 30 gal tank with nothing but a pump, a heater, live rock, regular tank light, and a Penguin filter, with a 'fill line' drawn on the glass to help you top off accurately with ro water from the supermarket. This is the ancient method. It will support a few crustaceans and snails and a couple of very small gobies or blennies or dartfish.

If you want more than that, your problem becomes too much nutrient in the system, not enough waste removal, not enough light for, say, corals, and evaporation getting out of hand. And water quality.

Can you get by without the filter? Yes. A filter is actually a problem, once you get beyond a couple of guppy sized fish and try corals. The simple Penguin is one of the better filters for this sort of thing because you never to rarely clean it---and consequently you don't have the up and down of the larger, more efficient filters, which break things down to Nitrate, which then builds up and up. If you didn't have a filter, the !!!!!sufficient!!!!!! live rock and sand would do the whole filtration job, including breaking Nitrate down to nitrogen gas, which floats up and away. So the first thing you should dispose of UNLESS you are going fish-only---is the filter.

A skimmer? Really good idea, the heavier your bioload, especially fish. Your corals are filters, and will slurp up a lot of biowaste, but the fish just poo, and a skimmer is the 'surf' of the tank: it creates froth of amino acids and you toss that. Some corals really have to have a super good one: sps stony. LPS and softies can get by without a skimmer on the small scale, but the more complicated your reef gets, the more you need at least a moderate one. A skimmer also is a major, major oxygenator, and your tank will be healthier for that!

A sump? Honestly, a downflow box and a sump are how you prevent your display tank looking like a mad scientist's experiment. YOu need space for all that equipment, and tucking it into a separate tank underneath (mine's in the basement, tank on main floor) lets everybody look at your fish and corals, not your equipment. A sump is also where you maintain a fuge. [refugium.] Which supplies free fish food, and oxygenates your tank.

An ATO [autotopoff] ---water constantly evaporates. This is a good thing, mostly, particularly if you want to raise stony coral (which uses kalk, which is driven by evaporation) but even if you have two clowns and a nem, you can use an ATO...so your tank doesn't become the Dead Sea from salt overload: salt doesn't evaporate: it concentrates, and without an ATO, your weekend vacation can have things in a sad state before you get back.

A reactor : these come in several types, but if you're having hair algae problems, or algae problems in general, a phosphate reactor (again, a good reason for a sump!) is a must. A fuge isn't enough to solve a really bad phosphate overload. A reactor can. You just have to remember the medium 'saturates' and has to be replaced during the course of treatment.

A ro/di filter: owning your own, even if you have to hook it up to sink or laundry, is a sanity saver, the bigger your tank is the more so. Tapwater safe for us to drink is not safe for fish and corals. And remember--your tank is a closed system with heavy evaporation going for years, so if your drinking water has a little arsenic (yes, some does) or other obnoxity, and you keep pouring in topoff---the nasty stuff just builds up and up. Remember, only WATER evaporates: minerals dissolved in it stay on and on. Your salt mix is perfect sea water ONLY if added to pure, no-mineral water, meaning ro/di, which has nothing, absolutely nothing in it. Otherwise you've got so-so seawater with a load of phosphate from local farmers, arsenic from your soil, and things with too many letters to spell here. Ro/di is pretty well essential, and remember---the ro/di filter cylinders are pricey: refilling them isn't. Once you have that, it will last you years.

A refractometer: of all things that will save you dead fish---this is one, fast, accurate, and easy to use. This is one thing you ought to have from set up onward.

A heater: get a good one. Bad ones are dangerous, and used ones are hyper-dangerous...A compromised heater can cause a fire.

Adequate test kits and supplements: alkalinity is a must for all tanks. For stony reefs, add calcium and magnesium. Get a test kit that's easy to use and read. If you have color blindness, Salifert produces numerical results with a very evident shift in opacity/light/dark of the solution.

Hope that will give you at least a checklist of what's good and not. A future reef does not need and should not have a big canister filter. A future fish-only can use one. Size of tank and purpose of tank all matter. Corals take specific lighting according to type: fish don't care much. You may not have an idea when you start just where you want to go, but start focusing and don't buy something pricey unless you know you need it: there are many ways to spend your money in this hobby. Ask advice about brand, size, and purpose, and it helps if you have some clue what you want to do. Coral reefs are no harder than fish onlies. Each has its hard and easy parts. The only cheap tank is a very small one with a couple of little fish...perfectly good tanks, those.