Is this your first time keeping a fish tank? ever heard of "nitrogen cycle", "new tank syndrome".....
If not please google those words first.....
I'm afraid u might ending losing your fish again & again later.....
For new tank setup, other than correct salinity your main concern is just to establish a complete & stable "nitrogen cycle" before adding any livestock........

Basically KH (carbonate hardness) functions is to stabilize PH from swinging upside down....
PH in a reef tank (coral) increase on daylight due to photosynthesis which consume CO2 and vice versa at night.....
PH swing is not good for marine livestock even though by 1 unit...
This is the reason why reefer dosing kalkwasser/limewater in their tank..to get stable PH....

For fish only tank, maintaining stable PH & KH is not a problem....

P/S: This is why i prefer to read more rather than listen to Fish Shop keeper.....
For tank of your size, mini chiller still can be effective....forget about it if your tank 2 feet & above....