im sorry to hear that!....when a tank crashes means that everything dies in a reef tank....can be all life or just fish or just corals. there are many things that will contribute to a crashed system.....mostly are due water parameters...do u check water parameters? eg high or low salinity can crash a system....high ammonia or high nitrites as well, if high nitrates some corals cannot stand the stress caused by high nitrates....u said white like spiders!...do u have any pics of these creatures!? cause they can be hitchhikers which are killing your corals!.....are the fish effected as well!?
water changes are good to be done....but make sure that the water that u are using is a good quality water...never do a 100% water change....always do partail water changes like 25%.
just a note....never loose faith...at the moment im as well have some downs in my system...mostly due high levels of phosphates and nitrates, which contributed in a ugly appearance of diatoms and cyanobacteria on my rocks.
if you post some pics of the problems it will be helpful to maybe identify the problem!
Goodluck
daniel