The pictures are depressing and certainly show that your colonies are pretty upset; however, I almost guarantee the destruction you see is a result of the high borate alk from the Seachem, the shift was to fast or the ALK test was not accurate. Why change something that works? Instant Ocean is good salt! I have often thought about changing salt but in the end never went through with it. Everyone’s reef and all the inhabitants get used to the chemistry of the tank regardless of the salt that is used, when a change comes to quickly is when disaster happens.
I also agree with Snappy, ZEOvit sponge power would be an excellent choice to help these corals heal but I would also add Coral Snow. Coral Snow doubles as a transporter or carrier of elements, within our water composition, easing their absorption [incorporation] by our corals as well as causing the skimmer to skim out extra toxins etc from the water column. Coral Snow has facilitated both of these functions very well for me.
Apart from one of the colonies you pictured that had obvious tissue necrosis, I believe given a little time and the right counter measures you will once again have a beautiful tank.
Here is a list of exactly what I would do should this situation happen to me, take it or leave it.
All dosages are formulated considering your water volume including your sump. Am I correct that you water volume is about 440 gallons?
~ Mix 18ml of ZEOvit Coral Snow with 18 drops of Sponge power. (Helps deliver elements to the coral more efficiently) Also add ZEOvit Coral Vitalizer 18drops per day alternating mixing with coral snow. These supplements all can be used on a non ZEOvit tank with excellent results.
~Let mixture stand for 5 mins then dose into high flow are of tank or sump. Do not turn off skimmer. Tank will cloud heavily for 2-3 hours.
~Change Carbon once every two weeks for the first month. Then once a month after.
~5% water change per week, any more than this may shift ALK too quickly. Sounds like you have already changed lots of water.
~Reduce photoperiod a couple of hours if possible for a couple of weeks.
~Adjust skimmer to skim very wet.
~Test for stray voltage
~Test for copper (never know!)
Confirm RO/DI TDS is ZERO!
Adjust water parameters to as close to NSW values as possible
ALK 7.5-8.5 DKH
SG 1.025-1.026
Temp 78-82 minimal fluctuation
K+ 380 (potassium)
CA 380-420
MG 1325-1500 ppm
PH 8.0-8.3
Nitrate 0-15 ppm
Po4 0 !!
I would be happy to give you some frags should you need to replace any colonies that won't make it.
Let us know the progress.
Details on products recommended by snappy and I.
CORAL SNOW
For small polyp and large polyp stony corals, soft corals and clams as well as problem solving/solution in a salt water tank.
Coral Snow is a liquid secondary biological facilitator for elements like B-Balance or Potassium-Iodide/Fluoride Concentrate. Compatibel with all elements, also for Amino Acid and Vitalizer. Coral Snow contains among others a natural Calcium-Magnesium Carbonate mix. With just 2.5µ Coral Snow has an optimal size for absorption by SPS corals.
We dose Coral Snow daily with another element each. The dosage can be put into a glass together with another product, mixed and after about 5 min. slowly into the streaming of your tank.
You can dose 1 to max. 4 ml Coral Snow per 100L/25gallons tank water daily.
Coral Snow neutralizes undesirable acids and is a solution against e.g. slime algae and cyano bacteria.
Therefore take Corals Snow and add 1 drop of ZEObak per 100L/25 gallons tank water each 2nd day.
The cloudiness in the water is harmless for all fish as well as sensitive corals.
SPONGE POWER
Enhanced growth of sponges and sea squirt support their water cleaning character and thus water quality significantly.
SPS corals get longer colored growing tips.
Especially purple is enhanced on all SPS.
The recovery of harmed corals is highly improved.
Dosing: 1 drop for 25 gallons or 1 ml for 500 gallons daily.