Freshwater or saltwater?

i have been wanting to buy a 14gal biotank for quite a while ive reaserched ALOT amd i have evrything planed out but i already have a 27 gal fresh water with some fishes (like 6) and i dont want to do the mistake of forgeting about them and that with time theyll die cause i rlly like em i dont know what to do!