I had to Google the Fritz 360 product as I don't use any of that type of thing. It's not a "chemical" per se; it's a dormant live-bacteria culture that is added as a "sludge remover" or aquarium cleaner. Their website specifically indicates that it is safe to use in conjunction with all other Fritz products, and detail that the heterotrophic bacteria that comprises it are "very safe even if overdosed". It sounds very similar to a product called, I believe, Rid-X which is intended to perform the same function in septic systems.
RD.
has a fascinating long-running thread on this stuff, well worth a read.
The idea of dechlorinator killing fish is something that I have heard before. There's at least one dechlorinator on the market (Prime?) which has instructions that actually recommend increasing the maximum dosage by five times in certain circumstances! Long and short, it would take a truly massive overdose to have a negative effect. It's also very difficult to believe that a major manufacturer would miss such a blatant danger in the simultaneous use of two products that are literally designed to be used in pretty much the same circumstances, i.e. new tank and/or new water. You won't find a more distrusting, more cynical or more "Show me!" person than me when it comes to manufacturers' claims and hyperbole...but I recognize that they need to protect their good name to keep their customers coming back. Both of these products are marketed as regular-use additives that they want you to continue buying ad infinitum...that would be tough to do if using them together, despite the instructions indicating this to be a safe practice, starting killing off tanks.
A comment made earlier and worth noting is that gar are air-breathing fish who derive much or most of their oxygen from atmospheric air; they would likely be the last to succumb to oxygen-depleted water even if that were the problem, rather than a cichlid like the Oscar. I grew up in an area where gar were very common; they live in places that few other fish can tolerate. The fact that the gar died and the oscar lived, to me, is a strong indicator that oxygen depletion is specifically not the culprit here.
Your buddy who runs the fish shop likely has a wealth of experience and knowledge...or maybe not...but either way, he can't really be expected to answer a question like this with "Gee...beats me!" as that would cast him in a very negative light. But, a lot of the time, there is simply no easy way to say for certain what happened.
I hope we get a few more opinions and suggestions from those reading this thread.
The idea of dechlorinator killing fish is something that I have heard before. There's at least one dechlorinator on the market (Prime?) which has instructions that actually recommend increasing the maximum dosage by five times in certain circumstances! Long and short, it would take a truly massive overdose to have a negative effect. It's also very difficult to believe that a major manufacturer would miss such a blatant danger in the simultaneous use of two products that are literally designed to be used in pretty much the same circumstances, i.e. new tank and/or new water. You won't find a more distrusting, more cynical or more "Show me!" person than me when it comes to manufacturers' claims and hyperbole...but I recognize that they need to protect their good name to keep their customers coming back. Both of these products are marketed as regular-use additives that they want you to continue buying ad infinitum...that would be tough to do if using them together, despite the instructions indicating this to be a safe practice, starting killing off tanks.
A comment made earlier and worth noting is that gar are air-breathing fish who derive much or most of their oxygen from atmospheric air; they would likely be the last to succumb to oxygen-depleted water even if that were the problem, rather than a cichlid like the Oscar. I grew up in an area where gar were very common; they live in places that few other fish can tolerate. The fact that the gar died and the oscar lived, to me, is a strong indicator that oxygen depletion is specifically not the culprit here.
Your buddy who runs the fish shop likely has a wealth of experience and knowledge...or maybe not...but either way, he can't really be expected to answer a question like this with "Gee...beats me!" as that would cast him in a very negative light. But, a lot of the time, there is simply no easy way to say for certain what happened.
I hope we get a few more opinions and suggestions from those reading this thread.