Twitchy Tiger Barbs and Shimmying Sharks

[sky.eyes.woman]

I've noticed something very weird with some of the fish we have for sale in our store. Tiger barbs (especially the smaller ones) and bala sharks are very often having this problem where they will be fine one minute, swimming around like normal, and the next they are twitching like they are in the grip of a grand mal seizure, unable to swim and then sinking to the bottom of the tank. Sometimes they recover in a half hour or so, and sometimes they die. They are always drip acclimated when they come in, we always feed them a variety of foods- Wardley tropical flakes as a staple, frozen bloodworms or brine shrimp or small tropical pellets. They have lots of cover in their tanks at all times. Sometimes I notice a fish 'seizing' for no apparent reason, the other fish in the tank are not chasing or bothering it, and no one touched the tank or stuck their hands in it. Other times being netted and bagged for a customer sets them off, or if we clean the tank and have our hands in there for a few minutes. Does anyone else see this in the store a lot or in their tanks at home? I'm completely stumped as to why this happens, anyone got an answer for this?
Since water params are asked for with posts like this, here they are:

pH- 7.2
Nitrite- 0
Nitrate- less than 20
GH- 75ppm
KH- 40ppm
Ammonia- 0
Chlorine- 0
Salinity is kept at an average of .2% (per corporate guidelines)

'Water changes' as you might expect are not done, the system has fresh water trickling in at about 1-2gpm at all times, with a dosing pump adding sodium thiosulfate with the water. Water is removed from the system every time we siphon tanks, usually 2-3 times a week.
This is the average water parameters, with slight changes in pH or GH on occasion.

 

[sky.eyes.woman]

*BUMP*
Anyone? Anyone...?
:look:

 

[Riso-chan]

Where is your stock coming from, are they captive bred or are they wild-caught? If they are wild-caught, the only thing that comes to mind is a possible poison(possibly cyanide) used to catch the fish. Eventhough I'm not really experienced in this, it does sound like they were exposed to something toxic, either that or they may be reacting to some unknown parasites. I'm sorry if I'm no help, but at least I can give an idea as to what it may be. I can't imagine what would cause a fish to do this, it sounds like something affecting the nervous system. Hope someone can help out.

 

[sky.eyes.woman]

I'm not so sure about the bala sharks, but I know the tiger barbs are captive bred. I will ask about it next time I see the manager. Thanks so much for replying...at least that gives me something to ask about so maybe we can figure it out. We lose a lot of both kinds of fish and I just wonder if there is anything we can do about it.