water changes are one of the most important maintenace things you need to do to keep your tank healthy regardless of the salt you use and what your dosing and it needs to be regular and around 10% a week is ideal . like myka said i too would go with h2Ocean i find it to be a better quality salt for a little more and a better quality salt goes along way . buy all the test kits you can and check for everything and post your findings here including temp and salinity. how much liverock do you have in your tank?? and what kind of livestock maybe something is eating your corals or nipping any new fish lately???
not sure how to tell if the bulbs are worn im sure someone knows a trick but i do know that if you dont know how old they are then you need new ones, if you have had this set for 1-2 yrs as stated and have not changed the bulbs you should asap, bulbs need to be replaced roughly once a year(8-9mths) if they dont your corals wont get their colour and grow properly and you may evenb end up with algae problems,
post some pics of the corals your talking about so we can see for ourselves plus i like to see others tanks lol
cheers