Aquarium Bacteria
Before adding the fish, there are a few essential procedures to check. To break down the waste products of the fish, a huge population of bacteria must be built up in the gravel, without allowing them to reach toxic levels. These wastes are broken down by the bacteria first into nitrites, which are highly poisonous, and then into harmless nitrates. There are two means of achieving the proper bacterial population, and with each you will need to use nitrite test kit. This is a special dye which you add to a sample of aquarium water; nitrite levels are indicated by color changes.
Method 1 - Use a commercial maturation kit, which has fluid which is dripped into the tank in measured amounts. It contains both bacteria and nutrients. Use the nitrite test kit over a few days until nitrite levels drop from 12-15ppm (parts per million) tops. Now a few small fish can be added.
Method 2 - This depends on a friendly aquarist or dealer. Add large scoop of gravel, complete with millions of bacteria, from well established marine tank to the new tank. Carry out the nitrite test as before.
The size of the population of nitrifying bacteria which develops depends on the amount of nutrients produced to feed them - that’s, on the number of fish in the tank. It is therefore not possible toad all the fish at once: they must be introduced only a few at time. It is sensible to add at first only small cheap fish, known to be tolerant of nitrites. Dascyllus are commonly recommended, but even more satisfactory are brackish-water or harbor-living fish like Scatophagus or Monodactyl, which can tolerate conditions that would kill most other marine fish.
Keep checking nitrite levels - daily for the first couple of weeks and then at weekly intervals. Increasing nitrite levels are a sign fan overpopulated tank, or of some drastic problem such as an undiscovered dead fish.