All fish stores sell tank setups containing ``everything you need'' for one price. However, a smart shopper looks carefully at what the package contains to be sure it includes only what you need (and doesn't include things you don't). Packages vary from store to store, some are more appropriate than others. Be especially wary of setups bought at discount stores (e.g., ``Hartz'' brand). They often include obsolete technology, noisy pumps, cheap heaters, etc.
Contrary to first impressions, larger tanks are not necessarily more work than smaller ones). In particular, it is easier to keep water chemistry stable in larger tanks than in smaller ones (the less water, the more easily a small chemical change causes a big change in relative concentration).
Much of the regular maintenance work does not require twice the time for twice the size. For example, a regular partial water change for a larger tank may require one more bucket of water than for a small tank. That doesn't translate into twice the work, since you already have the bucket and siphon ready, your hands are already wet, etc.
It is very common for people to really like their fish tank and want to add more fish. A larger tank can hold more fish safely. Indeed, a single 10g tank adequately supports only a handful of medium sized fish.
Note, however, that the number of fish that a tank can safely hold depends not only on the volume of the tank, but on its shape. For example, some fish spend their entire lives near the bottom. Doubling the volume of a tank by doubling its height won't allow you to keep more bottom dwelling fish. Surface area is more important than volume in determining how many fish a tank can support.
If possible, start with at 20g (or larger) rather than a 10g (or smaller). A 20g (``high'' or ``tall'') makes an excellent first tank size. Avoid all tanks smaller than 10g. They are simply too small to keep healthy. For example, although many stores sell them, the tiny 1 gallon goldfish bowls are totally inadequate for even a single fish. Stay away from them!
You will need a thermometer to verify that your tank stays at its proper temperature. Two types are commonly available. The traditional bulb thermometer works the same way as the ones you can buy for your house. They either hang from the top edge of your tank, or float along the surface. The second common design is a flat model that sticks to the outside of the glass. In this design, liquid crystals activate at a specific temperature, either highlighting the numerical temperature or a bar that slides along a scale.
Aquarium thermometers can be rather unreliable (check out the ones on display at a fish store --- they should all register the same temperature, but frequently don't). Thus, thermometers are good for verifying that your temperature is not too far off, but may be off by several degrees in some cases. When buying a thermometer, look at all the thermometers and pick one that has an ``average'' temperature, rather than one of the extremes.
There are three types of filtration: biological, mechanical and chemical. Biological filtration decomposes the toxic ammonia that fish produce as waste products. All fish tanks MUST have biological filtration; biological filtration is the cheapest, most efficient and most stable way to breakdown toxic ammonia. Mechanical filtration traps such particles as plant leaves, uneaten food, etc. (collectively known as mulm), allowing them to be removed from the tank before they decompose into ammonia. Chemical filtration (e.g., activated carbon, zeolite, etc.) can remove (under limited circumstances) such substances as ammonia, heavy metals, dissolved organics, etc. through chemistry (e.g., ``absorption'' or ``ion-exchange resins''). Chemical filtration is mostly useful for dealing with short-term problems, such as removing medications after they've served their purpose, or purifying tap water before it goes into a tank. A healthy tank DOES NOT require the use of chemical filters such as activated carbon.
Typical filters perform some or all of the three filtration types in series. Mechanical filtration (if present) usually comes first (where it is called a ``pre-filter''), trapping particles that might clog remaining stages. Biological usually comes next, followed by the chemical filtration section (if present). Whether or not chemical filtration is useful (or even helpful) depends on who you talk to. It can be useful for removing fish medicines after their effectiveness has ended (partial water changes do the same thing though). They can also remove trace elements necessary for plant growth (with obvious results). Unless you have a good reason to believe that your circumstances require chemical filtration, avoid it.
Gravel serves three main purposes. First, it serves as decoration, making your tank look nicer. Second, if using an UGF, gravel is mandatory as it is the filter media (the surface area on which bacteria attach). Third, in plant tanks, it serves as a ``substrate'' (e.g. dirt) for plant roots (consult the PLANT FAQ for details on what quantity and type of substrate is appropriate for plants). Ultimately, the choice of color, size, etc. is up to you. However, be aware that dark gravel better highlights a fish's colors. Fish adjust their colors to match that of the surroundings, and light gravel tends to wash out a fish's true colors.
Most of the gravel sold for aquariums is plastic coated. For obvious reasons, you should not boil it. :-) It is also very expensive ($1 a pound). Gravel can be purchased for much less at patio stores (e.g., Walmart, Home Quarters, local sand and gravel suppliers, etc.). However, it often tends to be larger than ideal and too light in color (e.g., marble chips). Sand can also be used.
It is safe to place items in your tank as long as they are inert, meaning they won't release (leach) chemicals into the water. Most plastics are inert inert, as are glass and ceramic.
Wood may leach substances into the water, changing the pH in a possibly inappropriate manner. Driftwood often leaches tannins and other humic acids into the water (much like peat moss), possibly softening it and lowering its pH. The water may also obtain a yellowish tea-colored tint. The tint is not harmful and can be removed by filtering the water through activated charcoal.
You will probably want to purchase lights and a hood. A hood prevents fish from jumping out of the tank and reduces the rate at which water evaporates. A good hood effectively seals the tank (except perhaps where the heater and filter reside). You want as little water as possible evaporating as it may raise the room's humidity to unacceptable levels and requires more maintenance (i.e., you will have to ``top off'' the tank once or twice a week to replace the lost water).
There are two styles of hoods. Full hoods combine the light and hood as a single unit. Hoods include space for only 1 or 2 (parallel) fluorescent light tubes, which is fine for fish-only tanks, but not usually enough for growing plants. Glass ``canopies'' cover the tank with two strips of glass connected by a plastic hinge, but don't include lighting. A separate strip (or other) light is used in conjunction with it. Canopies are a bit better for plant tanks than full hoods; one can upgrade or change the lighting without replacing the entire hood, and in situations where very high wattage is needed, one can usually fit more light bulbs directly above the tank.
Different types of bulbs give off light in different spectral regions. So-called ``full-spectrum'' bulbs attempt to reproduce the sun's full spectral range. They are good both for growing plants and bringing out a fish's natural colors. Specialized ``plant'' bulbs (e.g., gro-lux, etc.) emphasize a spectral range that stimulates plant growth. Such bulbs grow plants (and algae!) well, but fish don't look quite right under them, because the light does not have the spectrum of normal sunlight. The common ``cool white'' bulbs give off light designed for humans in windowless offices; they neither grow plants particularly well, nor bring out a fish's natural colors. As a quick rule of thumb, 2-4 watts/gallon of full-spectrum (or specialized ``plant'') lighting is good for plants; for fish-only tanks, use less than 1 watt/gallon, and avoid using plant bulbs.
A powerhead is a water pump that runs completely submerged in a tank. They typically attach to the ``lift tubes'' associated with UGF filters, pulling water through the lift tube. The stream of outgoing water can usually be oriented in (almost) any direction, and it is common to point them in such a way that water circulates throughout the tank and stirs up or ``agitates'' the surface a bit.