losing SPS colonies fast!!!

i am battling this problem for 6 weeks now and i have run out of ideas what is causing it. i have lost at least 8-9 big colonies of SPS from garf bonzai to sunset millie's, tri color validas, prostratas and some high end colonies and also all of my australian sps are gone.
this is how my tank looked like before everything went down.

everything is in check, water parameters are in check. nitrates, ammonia, phosphate are all 0. used 3 diff. test to confirm and all came out zero. dosing kalk and magnesium ever since. temp constant to 78. tried to watch for predators at night and nothing came up.
everything started about 6 weeks ago i noticed that my wall hammer was having brown jelly disease. took it out and dipped in iodine solution and was put back in the tank. a few days later noticed that some of my corals are losing their color and turning brown. next thing you know STN and RTN are all around the tank. since then i have done at least 10 water changes of 30g each time and since have slowed down the problem.
woke up this morning and just lost a prostrata (this colony have had RTN ever since but recovered from the water changes) and saw this.

this colony has been damaged with RTN since start but has recovered but woke up this morning and the RTN is progressing.
any one have any ideas?
i have run out of and tried to do a lot of research from the web. could it be a bacterial disease attacking my sps?

on the side note some of my sps are healthy and thriving. but most of the big colonies are gone. this is puzzleing for me.
any help is appreciated.

 

Parameters and maintenance schedule?

 

give us the specs on the system; equipment, maintenance schedule, chemicals used.

 

cal. - 480
alk - 9.5
ph - 8.1
mag - 1450
nitrates - 0
phosphates -0
ammo- 0
nitrites - 0

water changes every other week 30g with reef crystals.

 

Do you run GAC, GFO? Do water changes? How much salt, how much, how often? Do you monitor ca and alk? How often? have you changed test kits?

Most likely it is parameter swings, such as alk. Or, could be buildup of something not tested, such as organic carbon. Also, could be accumulation of metals or other toxins. etc... We need more info really.

 

90g with 30g sump. no chemicals used except for kalk and mag dosing.
swc 160 skimmer, ai sol led.
one thing i noticed is my chaeto's are dying but my gracilaria is thriving so that even it out.
one thing odd is i am growing this green slime on my return pump compartment of my sump. it looks like cyano but is green color. dunno if that has any link to my problem.

 

AEFW or some other predator?

 

What is your Mg at? What is your specific gravity/salinity?

 

yes i have GFo and carbon running. Carbon is 1 cup and GFO is 2 cups. carbon changed every 2 weeks and GFO q month.
if it was parameter swing it should have affected all my sps/corals but this one is targeting my big stablished colonies.

 

Has anything new been done to the tank lately?
IE new chemicals/new batch of chemicals
new salt mix/brand
new light bulbs/brand of light bulbs
ANYTHING that has changed in the last 6 weeks

I had this happen on a customers tank before and it turned out to be a bad batch of reef chemicals that he had just purchased, I dont know if someone put something in the bottle at the store or something was wrong at the facility but we bought a new brand of chemicals and did a 50% WC and within 2 days the RTN went away and after a few months you couldnt even tell anything was ever wrong with the tank