A case of Ich

If the fish(dead) are disposed of promptly, there shouldn't be any effect on the other inhabitants as far as spikes in ammonia, nitrite, nitrate and phosphate. Check though for ph and the others listed above to be safe. Alot of times you can have a good indication of water quality by how well the corals are doing, or other fish and inverts.
A water change would also be helpful in establishing pristine water!

 

Orange_Crates,

Since you are a guest and it's not your tank, I'll make my comments brief:

-- What you described is approximately a 200 gallon tank.
-- The fish are apparently stressed, either because of the recent move, or because they need more room (A self-correcting problem if some of them die).
-- Your grandparents should continue to check the water parameters -- temperature, pH, salinity (specific gravity) and levels of ammonia, nitrite, and nitrate every day or two. Weekly, they should change out 10-20% of the water.
-- Wait and see if things get better.

 

The inverts and coral are doing great (well...except when they get eaten), so I guess the water is okay?

...I think my estimation might be off. Funny, it's usually quite accurate when I do the estimation in centimetres...

 

if sufficient decaying mass (in english if you leave the dead fish inside the tank! LOL) are present in a tank, this will pollute the tank and cause more disasters.
remove dead or decaying mass ASAP. whether it be food, dead fish or dead/decaying corals.

btw sorry about your troubles. be happy though. everyone goes through it (and anyone who did not is a big fat liar...hehehehehe)

 

Since nobody wants to try reef safe meds, tell your grandparents about a UV Sterilizer. There might have a chance of some fish living.

 

there are medications that you can put in a reef tank just find a shop who actually care about their livestock and liste to any advice they give because you will get agreat deal of pleasure from this hobby if you are willing to spend time on it

best of luck