There are 3 distinct major types of saltwater aquariums you can own, depending on what look you want for your tank, level of complexity you can handle and what budget you have.
⪼ Fish Only
⪼ Fish Only With Live Rock (FOWLR)
This type of aquarium focuses on marine fish and will generally have no or very few invertebrates, although hermit crabs and snails are very helpful to deal with any algae growth.
Dead coral skeletons or fake corals and rocks will form the background to your fish only set up.
Fish only aquariums are the cheapest saltwater aquariums to own as you do not need elaborate lighting, rigs or water and phosphate filter set ups.
There are 3 main subtypes for your fish only aquarium:
1. A community based aquarium inhabited by peaceful fish species that get along well together for example small groups of small herbivorous species.
2. Semi-aggressive species that should be kept one per tank. In this set up you will have fewer fish as these fish tend to be larger.
3. A biotope fish only tank, which has groups of fish focused around one or two main individuals from a specific geographical location such as a Fiji island lagoon for example.
Fish only set ups require more regular water monitoring and partial water changes because you will need to reduce accumulating nitrate levels in the tank. Live rock will help naturally reduce nitrate levels for you.
You will need to periodically monitor the pH, ammonia, nitrite and nitrate levels. These readings will give you a good handle on the water quality inside your tank. It will also give you an idea of how regularly you should be performing water changes.
A FOWLR is essentially a fish only aquarium that has the addition of live rock.
Live rock is rock harvested from the reef rubble zones of the ocean; it contains living on and in it a myriad of tiny invertebrates and beneficial microorganisms.
Live rock when cured and put into a saltwater aquarium initiates the nitrogen cycle, which will uses 3 different populations of bacteria to convert waste ammonium (from decaying uneaten food, detritus and marine life excrement) into nitrite, then nitrate, then harmless nitrogen gas.
This stops these chemicals building up to toxic levels in your tank and killing your marine life.
Live rock and live sand will be your biological filter in this set up and thus greatly aids water quality and increases stability.
Not only this live rock looks natural and cool in your tank and also gives your fish places to play and hide and even provide morsels of food.
There are a variety of interesting hitchhikers (some you don’t want that will need to be removed before your live rock goes in the tank) that will help with cleaning duties and generally add to the interest and color of your aquarium.
Live rock may well be the the most expensive part of your FOWLR as you should aim for 1 or 2 pounds per 1 gallon of water, but it is well worth it in my opinion.
FOWLR set ups usually are really good to keep a few invertebrate species in as the live rock provides a fantastic eco system for them.
The beauty of a FOWLR set up is that is can be easily converted to a reef aquarium later on by upgrading lights and adding a few other bits of equipment.
A reef aquarium has the emphasis placed on corals and other invertebrates rather then fish.
In fact there are usually only a few reef safe fish (non coral/invertebrate eating) in a reef set up to help keep the nitrate (>10ppm) and phosphate (>1.0ppm) levels low, which corals and other invertebrates do not tolerate.
The reef tank is the most advanced and expensive set up but also the most visually stunning.
Corals and some invertebrates can be challenging to keep as they have specific lighting, water quality and water movement requirements.
The lighting set up and high quality water supply (reverse osmosis or deionized) are the expensive parts. Certain corals and other invertebrates can also be quite expensive, so you need to know what you are doing.
A fair amount of experience is recommended for owning a reef tank, because you will need to keep the water parameters perfect for corals and invertebrates, which means a lot more regular water quality testing.
You will also need to know exactly what lighting requirements, food, preferred position in the tank and water movement levels your chosen corals and invertebrates require in order to have them thrive.
You will need to have plenty of live rock for biological filtration and will need to add supplements to the water for the corals such as calcium, strontium and iodide to keep the corals happy.