Unusual death. Help solve this mystery.

[atty]

A week ago i purchased two sleeper gobies, they were all doing fine until one started to have a fit yesterday, it started swimming into things untill it just flipped extremly fast and suddonly died. This also happened 2 nights ago with a different gobie, it died the exact same way. And tonight my bi-colour is doing the same thing the sleeper gobie did 3-4 hours before its death. The bi-colour is gasping for air and is hiding behind the rocks, all the other fish are coming out and look normal. These fish were all healthy and fat 6 hours before there death. Im thinking these fish could of swam into a coral and got poisoned or the temp is to high (tank is 29-30 degrees). The tank has been set up for a year or so and its a 4 by 2 by 2 and has a l00ltr sump/refugium making it a 500ltr total volume.
inhabitants
1 fox face
1 bi-color angel
2 purcula clowns
1 sleeper gobie (were 2)
1 bi-colour blenny
1 green finger coral
1 leather
1 Catalaphyllia
1 frogsprawn
1 zoanthid colony (the one with the tree like structure)

Ammonia 0
nitrite 0
nitrate 10mg/l
Do you guys have any idea what this could be?

 

[atty]

sadly my bi colour just passed away tonight, i tried to get him in a bucket with sterazin but i was to late, just as i put the medication in the bucket he started to fit so i know he was gone. I dont know what i can do to save the rest of my fish but i think my fox face has caught this parasite. Ive just soked some brine shrimp in garlic so ill feed my fish with it tomorow.