The first thing you need to know about saltwater tanks, is that you can not have nearly as many fish as you can have in a freshwater tank! There is not a "rule of thumb" for stocking an aquarium, so I won't even try. The fish load is determined entirely by the type of filtration, quality of aquarium maintenance, and types of fish you want keep.
You should also be aware that the types of filtration recommended for a saltwater aquarium are different than for a freshwater aquarium. In freshwater, the goal of the filter is to break down Ammonia into Nitrate. In a saltwater tank, we would prefer to directly remove the organic wastes from the water column, before it has the opportunity to become Nitrate. The reason for this is that Nitrates are not tolerated by saltwater fish as well as freshwater fish. Due to this, if we used the same biological filter on saltwater that we use on freshwater, we would have to change extremely large amounts of water frequently.
My goal in setting up a saltwater tank is to make it as "hands-off" as possible. This can be accomplished by setting up a natural system in which all the waste produced is completely utilized by the system. This is actually not all that difficult to accomplish. Regardless of whether you are setting up a reef tank or a fish only tank, the setup should be almost identical. Even if you only plan on keeping fish, the aquarium should be set up as natural as possible, hence the reef environment. For every saltwater tank I recommend the following equipment: live rock, aragonite sand, and a protein skimmer. In addition, if you are keeping a reef tank, you will want to add the proper lighting. If you are keeping a fish only tank, you will want to add a UV sterilizer.
The aragonite sand and the live rock will provide the natural setting for your fish and inverts. The copepods, amphipods, and other small life forms will migrate from the live rock into the aragonite sand over time. This will result in "live sand." It is not necessary to purchase live sand from the start. The live rock and aragonite sand will provide all the biological filtration necessary for a fish or reef aquarium. An additional biological filter is not necessary. The aragonite sand bed will also serve as a denitrifying filter which will break down Nitrate produced in the aquarium, provided you are not introducing an unnatural amount of Nitrate by using a biological filter. The protein skimmer will remove the bulk of the organic waste. The remaining waste will be metabolized by the live rock and live sand bed. This is the ideal setup, as all waste is being utilized by the system naturally.
The UV sterilizer should only be used on a fish only tank. The UV bulb will kill pathogens and parasites as they flow through the unit. The result is an aquarium which is much healthier for the fish. The UV allows your fish a greater chance of avoiding disease. I can not emphasize the benefit of a UV sterilizer enough. Although many hobbyists have success without the use of a UV and would consider this an optional piece of equipment, I personally think it is worth the small expense to provide this additional "insurance" of sort. I will never again set up a marine aquarium without a UV sterilizer.
For a reef tank you will need to provide the proper lighting for the corals you are planning to keep. I will not go into a lighting discussion, as there is an unbelievable amount of material which is widely available on lighting a reef aquarium. The lighting issue is debated by experts and beginners alike. Some of the more commonly used lighting systems include Metal Halides, VHO's, Compact Fluorescent, and regular watt daylight bulbs. I personally lean toward the use of VHO's due to the ease of use and availability.
In addition to the above equipment you will need a heater, a salt mix, a hydrometer, and test kits. I keep my temperature at 25°C, my salinity at 1.024, and I currently use Instant Ocean salt. I am strongly considering a switch to Marine Environment salt due to recently released information. I test my water for Nitrate and Alkalinity on a monthly basis. Your Nitrate should be non-detectable for a reef tank and under 20 ppm for a fish tank. Alkalinity is best used to monitor trends in the buffering capacity of your aquarium. This can help you to determine how frequently you should be changing water.
As a final note, I strongly recommend that you read as much material as you can find about saltwater aquariums. Check out the Saltwater Books and Site Links available on this site. Good luck!
Another tip about setting up your aquarium, if you don't want misty water when you have to throw in salt, throw it in the protein skimmer. It will speed up the dissolving of the salt and keep your water clear.
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