It was an existing tank, not too sure of the make but it is one that you can buy as a complete set up. It is small probaly only about 55 liters.
OK. So, if you bought it as an existing setup I assume that it is an established system and that the tank cycle has been completed.
Would be good to know what make of tank it is ( Boyu , Nanocube , JBJ etc.)
It is important to look at the following basics :
1. Substrate used should be fine particles of not more than 1.5mm in diameter. Playsand or Aragonite substrate works well .
2. Mechanical filtration should be by means of a skimmer , filter floss/sponges
3. Good quality Live rock . The emphasis here is really on the quality and not just the quantity. This forms your biological filtration and is vitally important.
4. Carbon and Phosphate removing compound. This completes the filtration process by addressing the chemical side of things. Carbon will furthermore "polish off" the water .
Some investments to make:
Test kit ( pH , Nitrate , Ammonia , Nitrite) to start off with
A good quality hyrdometer ( reftractometer - a bit pricy , swing-arm type , floating)
For Nano tanks , regular waterchanges of 10-15% are crucially important.
For top-up use only RO water .
That covers the basics ( hope I haven't left anything out).