Here are a few descriptions of generic aquariums which may help you get an idea what is possible:

  • "Fish only" with fake decorations: The real emphasis is on the fish. Lighting which enhances their color and decreases algae growth make these tanks generally easier to maintain. Colorful plastic corals and rockwork made from artificial materials decorate the tank. Other aquarium safe artificial decorations such as a treasure chests, the Colosseum, skulls, etc. are fine. There are a few invertebrates which may work in this type of tank including hermit crabs, starfish, and sea urchins. These animals may be eaten by fish like triggerfish and pufferfish and are not able to remain in the tank when most medications against fish parasites are applied (a major advantage of a fish only tank vs the other types).

  • Fish only with "live rock":the rock is from the ocean and grows some algae and houses things like worms snails and seasponges. These tanks can also accomodate a variety of invertebrates including the full cleanup crew" type of assortment of snails, hermit crabs, etc. provided again that you are not keeping fish which eat small invertebrates (some wrasses, triggers, puffers, etc).

  • Fish with live rock and a few durable corals like mushroom polyps or leather finger corals. With the right light these aquariums are the easiest type of reef aquarium to maintain.

  • Reef tank with durable corals, a variety of fish, and larger colorful invertebrates like starfish and shrimp.

  • Reef tank with less emphasis on fish and more focus on dense diverse coral 

A highly regarded aquarist once gave a lecture on how killer whales are reef aquarium safe because they don't eat corals. Understanding how these tanks work allow for you to take matters into your own hands and break the rules by adding more fish than recommended, keeping some corals and invertebrates in tanks where at first glance a fish store employee would recommend against it, or mixing species which may traditionally not get along.

Example aquariums:

It's all about the fish: 125 gallon aquarium with a wet dry filter, a mediocre protein skimmer, and  600 gallon per hour powerhead. A t-5 double bulb florescent light strip which has one white light and one blue light. The decorations are base rock and plastic corals and plants. You can even have a replica of the titanic with bubbles coming out. For fish: an emperor angelfish (or any other large angel), a powder blue tang( or any other tang), a flame angelfish (or any other dwarf angel), a pair of ocellaris clownfish( or any other single or paired clowns), a foxface, 1 or 5+ anthias (any type), a purple psuedochromis (strawberry fish), a porcupine pufferfish (or any other pufferfish or small triggerfish), and a coris gaimardi wrasse (or any other larger wrasse). Not having any invertebrates or coral will allow you to treat this tank with medications if ever there is a disease outbreak.

Predators: Same tank and equipment as the previous aquarium. For Fish: A lionfish, a miniatus grouper (or any other grouper), a zebra moray eel (or any other eel which stays under 3ft), 1-2 different pufferfish, 1-3 different triggerfish, large queen angelfish ( or any other large angle fish), a large clown tang (or any other large tang).

The diversity of the ocean: 55 gallon fish tank with a sump, a good protein skimmer, and 2 x 400 gallon per hour powerheads. The light can still be a double bulb t-5 florescent but a higher quality light (4 x t-5s or LEDs) would keep live rock growing more healthy organisms. A chocolate chip starfish, a coral banded shrimp, and tuxedo urchin as well as 25 algae eating astrea/lithopma/ margarita/nerite snails and 25 blue legged hermit crabs. Pair of ocellaris clownfish (or any clowns), 1 medium tang (yellow, yellow eye, hippo), 1-2 cardinal fish, 1-2 damselfish, a coral beauty angelfish (or any dwarf angel), and a sand sifting goby (bullet/rainford's/hector's).

A complete ecosystem: 55 gallon fish tank with a sump, a good protein skimmer, and 2 x 400 gallon per hour powerheads. A reef quality light. All live rock as structure. Reef safe fish: A foxface (or medium tang), a pair of clowns, a six lined wrasse ( or fairy wrasse), a neon goby (or other small gobies), and any reef safe inverts you want to pile into there. Sand sifting starfish, all of the clean-up crew options for snails and hermit crabs, coral banded or any other cleaner shrimp, reef safe sea cucumbers and brittle starfish, etc. Corals will grow with good water quality and the appropriate light. If all of these other organisms are thriving and the live rock has a lot of sponges and worms growing out of it, you can grow about any coral which is photosynthetic. The key with this tank is build up slowly. If you put all of the fish in right away, their food will come out as fish waste and create a poor water quality before live-rock inhabitants (the sponges, worms, and other growing organisms)  can build up to absorb and use that. Then the poor water quality makes corals not happy, they may die, and it's a downward spiral. Start slow, build slow, be successful. Initially be very conservative with the fish, for the first 6 months maybe only have 1 or 2. When you see lots of worms and sponges growing from your rock, that is when you can try to add a few more fish.

Corals focus: High quality lighting and diverse flow patterns, often controlled by an aquarium "controller" is ideal for this type of aquarium. A skimmer is a good idea but the general rule is start slow and do not put too many fish in. A few small reef safe fish ensure that the nutrient levels will stay low and not irritate corals throughout the exolution of the tank. Adding a few more fish later, as the bioload increases, is usually OK but the ultimate demise of many coral tanks is a downward spiral where something small died and created a nutrient spike locally which killed a few more things which released more nutrient etc.