i have a 55 gallon fresh water that i want to convert to salt water spoke to the aqurium place about what i had and they said that filters that i have are fine just needed salt and live rock. was wondering if i needed the live rock right at startup or if i can add later. and how much i would need.

Hey there,
Live rock usually is the primary biofilter for the salt water tank. it has to go through its own cycling also. so its best to start it from the begininng. Usually people will tell you to put in at least 1 pound of live rock per gallon. You probably should do a bit more though. Also, sand will provide a good substrate for biofiltering also. I've been told by others on here that a normal HOB filter isnt the best choice. Refugiums and sumps seem to be the leading filtering method outside of the live rock.