!!!! FISH SEIZURES !!!!!! pLEASE HELp!

[Nexus13]

So I've had my tank for about a year and a half now.

Lately some crazy thing have been happening.

So background for my 29 gallon tank with 10 gallon sump.

Tank Occupants:
Clown Fish
Yellow Tang
Clam
various corals
Mandarin Goby
Cleaner Shrimp
snails and hermits
2 x peppermint shrimps
various macro algaes in sump

New additions
Pinstripe Wrasse
3 x poreclain crabs
neon dottyback

Equipment:
Aqua C Remora hang on back protein skimmer
CPR overflow
2 x T5 HO lights on main tank

Readings:
Ammonia = 0
Nitrite = 0
Nitrate = 0
pH = 7.9
Salt = 1.022
I dose with purple every 3-4 days.

What's happening:
In months prior, I've seen my Yellow Tang go into what seems like an Epileptic seizure. It will fly across my tank knocking corals and bumping into glass walls for about 10 seconds. Then it will lay still wherever it ends up for about 30 seconds with all its fins flexed, and just breathe without moving. Then as if flicking a light switch, it will come back to life (although quite lethargic) and hide in his sleeping spot with his fins all droopy for about 10 minutes, after which point he is swimming and eating again like normal. I looked it up and tried to find any info i could on it but nothing seemed to fit my description of the problem. I've seen this happen a total of 4 times now over a period of a 6 months. Weird thing is that none of my other fish (clown, mandarin goby) have ever had this happen to them, or that I've seen.

So anyways, I came to the conclusion that my fish was epileptic and had accepted it for that. Now what's scaring me is that after a recent addition of the creatures listed above, 2 porcelain crabs have died (after about 2 days of being in the tank, they aren't swinging their little filter feeding claws at all, just sitting in a spot), and today (2 days in), my pinstripe wrasse had a seizure. His seemed much more violent and he was also gasping at the top of the tank and spazzing. He laid upside down on the sand just breathing for about 5 minutes. Now he is slowly "coming to". He doesn't look nearly as lively as the tang did after his shock. I'm very concerned as to what could be causing these seizures and why my crabs are dying. The neon dottyback seems very healthy and has had no issues.

I'm using tap water for water changes (decholrinated with Prime) and have been doing them, mostly every 2-3 weeks, for the past 3 weeks i've been doing it weekly. I feed my fish mysis/brine shrimp/and spectrum.

I don't think its an electrical issue because then all my fish would be reacting if it were.

Has anyone heard of such a problem?

thanks in advance for your help.