Although I use and recommend highly some API products, StressZyme is not one of them. I personally see no benefit to this product, and I do think it may be detrimental. It will not likely have any impact on nitrite (or ammonia) issues.
Salt. This can help nitrite issues. However, salt should never be used in an aquarium with freshwater fish species (with a very few exceptions) long-term such as a sort of "tonic" because it does harm to fish. It can be very beneficial as a treatment for some problems (ich, for one) but this is as a treatment for a specific issue. I don't know if you have been using salt all along, but if yes, that may be part if not the issue with the clown loaches dying and now the RTS. Loaches absolutely cannot tolerate salt continuously. But as I said, if you see nitrite above zero, this can help, but it should not be continued and hopefully the nitrite will resolve itself.
Water changes are essential here; significant volume, meaning 70-75% of the tank volume, daily if nitrite continues above zero, but otherwise I would do them alternate days for a week. [Note that Colin suggested much the same.] As a general rule, whenever I see something out of the ordinary, I do a major water change; it is interesting that sometimes that is almost all one needs, depending. Water changes are without doubt the most effective and beneficial task whether regularly or as here for issues.
Which conditioner? Not likely to be a problem, though there are a couple that are not the best because they contain additives that can be detrimental. But with ammonia or nitrite, a conditioner that also detoxifies these can be helpful until they no longer show in tests. But aside from that, I would use the simplest conditioner, as all you really need is a dechlorinator (for chlorine and/or chloramine). API's Tap Water Conditioner is what I have been using for a few years now, and highly recommend.
