I'm still a novice at keeping fish myself, but being that you have no answers so far, I'll give it a shot:
You didn't mention how much water they're in or if there are other fish. If there are only 6 tiger barbs in the whole tank, then yes, two pinches sounds like an awful lot to me. We have about 20 fish and do one pinch, and they do fine. I've read that fish can go for days easily with no food and probably don't even need food every day, but many people don't know that, so over-feeding is very common.
On the other hand, I know a spike in nitrate soon after starting a tank is a normal part of a cycle, and that it will soon rectify itself.
I would contact a couple of pet shops around you that specialize in fish (or who have a person specializing in fish) and ask them, to make sure.
Note after seeing my answer as number 19 (!) : When I saw your question, it said it had been asked 3 hours ago and there were no answers. That's why I said what I said.