I keep marine, have done for years now.
Do not put them in after a few days, they will be dead in no time.
The tank needs to cycle, it needs to go through the bacterial blooms and then settle.
you need 4 kits to start
PH, Ammonia, Nitrite, and Nitrate.
Only until there is no trace of nitrite and ammonia (especially ammonia) and low levels of nitrate (some people would not want even nitrate), but I find personally a very low amount of nitrate for me is better than none.
Then you have phosphates, calcium, magnesium - depending on what you are doing these are also very important!
Do not get sucked into putting fish in mate to put smiles on little faces lol... it will end in tears!
Mick