Have only heard of issues with Bacter AE causing deaths... there may be other products as well, but I can't say. Bacter AE is one of the more popular brands that people purchase, so it could be because of that as well.
I hear of more people using a *tiny* amount of Bacter AE over those that use the suggested amount. If someone is having random shrimp deaths and Bacter AE is one of the foods they feed, then completely cutting that out and/or increasing oxygen is one suggestion to see if the deaths stop.
Shrimp Fit appears to basically be brewers yeast with added minerals, so you should be safe there. Feed in small amounts.