You may need a heater for the aquarium. Whether or not you need one depends on how warm your aquarium will stay without a heater. If you can maintain at least 72° F (22° C) you can usually get by without one. Lighting systems will transfer some heat to the water, so even though the room temperature is only 70° F (21° C) your aquarium will usually be several degrees warmer. Make sure you invest in a thermometer.
You will need a hydrometer to measure for salt level content. Marineland™ makes a new re-designed hydrometer that I recommend for Nano/Mini reefs. We try to maintain a specific gravity (salinity) of 1.023 to 1.024 in our aquariums. Instant Ocean® and Reef Crystals® are our preferred salts. You should first add water and salt to your aquarium and let it mix overnight with the power filter running. The next morning test the salinity. Add more salt to raise the salinity if necessary, or take out some water and add fresh water to lower the salinity. Let it mix another hour and test again. Once the salinity is at the desired level you can then start adding livestock.
There are 3 trace elements you should add on a regular basis. Calcium, Strontium, and Iodine are the trace elements we add at least once a week to our reef tanks. Calcium and Strontium are added for the stony corals, as they will strip these trace elements out of the water for there stony skeleton, and Iodine is needed for shrimps and crabs, so they can molt properly.
The best substrate for the reef aquarium is very fine aragonite (coral) sand at a depth of 2 to 3 inches (7.5cm). A 50-pound bag covers 325 square inches at about 3 inches or 10 pounds will cover 65 square inches. A typical 20-gallon would require about 45 pounds, and a typical 10 gallon would require 30 pounds. Why the fine aragonite sand? The number one reason is, once microorganisms colonize the sand it will start reducing nitrate (NO3). The lower depth of the sand bed houses anaerobic bacteria that use the oxygen atom on the nitrate molecule to breathe. The sand will slowly be turned over by small organisms that live in the sand bed. This will allow the water to slowly be exchanged in the lower sand bed, allowing an anaerobic condition to exist. Large forms of gravel will allow to quick of a water change over for an anaerobic condition to take place. It can be done, but it would require an extremely thick bed.
Live Rock and/or Live Sand and/or Bio-Spira from Marineland should be added to the aquarium after the salinity has been set. This will help seed the aquarium with beneficial bacteria that will break down ammonia and nitrite. The base Live Rock should be in contact with the bottom of the aquarium. Once the base Live Rock is in contact with the bottom you can then pack your sand around the rock. This is done to help prevent any animals from undermining the rock and then creating all the rock to shift and possibly tumble down, risking the tank breaking or getting scratched. Live Sand should be added on top of the dry sand. You want the beneficial bacteria in the live sand to be in contact with the water column.
Once your aquarium is set up it is normal to go through algae blooms. The first algae that shows up is a brown algae, then usually followed by green hairy algae. Algae can be controlled by adding 4 Astria (Florida Turbo) snails per 10 gallons.
I prefer to cycle the reef tank with crabs and shrimp, rather than fish. They tend to be less of a bio-load on the aquarium and seem to tolerate ammonia and nitrite better than fish.
Mini (Nano) reef tanks are a very good way to get your hands wet in the reef hobby. Contrary to what many people have said, small reef tanks are very easy to keep. Many of the animals are very hardy, and will live for many years.