Hi and welcome to the addictive hobby of marine fish and reef keeping.
First off let's talk cycle. Fish wastes, uneaten food, detritus in tank all turn into either Ammonium or Ammonia. This is incorporated (eaten) by bacteria and changed (pooped) into nitrite. This is yet again incorporated by another bacteria and changed (pooped) into nitrate. Nitrate is then incorporated by yet another bacteria and changed (pooped) into nitrogen and oxygen. NH3
> N & O2
. This is rather over simplified but it will paint a better picture.
When a new tank is started there is actually a pretty sterile environment.
We introduce some kind of fish food (small amount) and the first bacterial populations begin to grow. They overpopulate based on abundance of there food source (ammonia). As they stabilize in growth their wastes (nitrites) are food for another group of bacteria which begin to overpopulate. This new group of bacteria goes through yet another population explosion feeding on nitrites and leaving nitrates. If you have a deep sand bed or plenty of live rock yrt another bacteria will form deep in the rocks and under the sand where there is little oxygen and utilize the O2
off the NO3
to live. In brief this is the nitrogen cycle and in most systems it will take 28 to 35 days to establish.
Now your tank will go through other chemical and algal cycles to balance out in about 100 days. Fish should never be added into a new system until first nitrogen cycle is complete. Use test kits and watch for ammonia spike, then nitrite spike then nitrate spike. After first month it's time for water changes to begin so excess nitrate can be exported out of system.
Macro algae and or live rock can aid this alot. Point being no fish ever before about day 36 - 40. Then only ONE or TWO small ones. At this time you may also introduce a clean up or janitorial crew.
Test kits are a must, never add to a tank what you don't test for.