I thought it might have been an amonia spike. I almost had the same problem moving water and sand from one tank to another. What was weird is that my amonia was 0 but my nitrite was pretty high. I had to keep the fish in buckets and add a amonia, nitrite and nitrate reducer. That didn't help until a few days. I was worried about my fish making it. Sand is great when it comes to getting waste off the surface, but it also traps more then I thought it would.