You should only be dosing if your calcium , alkalinity , and magnesium are low , to indicate if u need to dose you first have to check those levels , i have a 20 long mostly lps softies and a clam and my calcium never drops below 400 so i dont dose anything if it was 300 i would have to dose to bring it up but i do weekly water changes and ive never testes my calcium below 400 and my alk is always 7 or 8 so no need for dosing it all depends on the system you have to test
I have a 24 gallon with LPS and they grow fine with no dosing. Just feed them and give them good light and they will grow. You water changes should take care of everything if it is a small tank.
There's no need to dose a small cube...your making more work for yourself that isn't needed...you only have softies and lps.regular water changes is more than enough to replenish dissolve nutrients,I have a 34 solana with mostly sps and I don't dose anything just weekly water changes