If your nitrates are running that high and not coming down with water changes it's probably a good idea to try this product or some other form of carbon dosing and think of it more as a slow, long-term solution than a quick fix (this product will work fine, but regular white vinegar or vodka might be a cheaper long-term solution FIY).
The fish deaths you're reading about are probably being caused by people dosing too much too quickly. That'll cause a huge bacterial bloom which will deplete oxygen from the water and suffocate the fish. To keep that from happening you need a protein skimmer running 24x7 in your system (or, at a bare minimum, lots of surface agitation from an airstone). It's also best to dose it while the tank lights have been on for a little while because that's when oxygen in the water is highest.
Start the dosage slow (to be safe do something like half of the recommended dose on the bottle and ramp up over the course of a month or two by increasing the dose a bit every few days. The goal is to very slowly build up beneficial heterotrophic bacteria in your tank by feeding this product. Over time they'll bring your nitrates down and eventually once you get your nitrate levels in check, you cut back on the dosage (by maybe half or so) and continue to dose it indefinitely as a maintenance dose to keep the bacteria you built up from dying off and causing nitrate to rise again. It's a great solution as long as you think of it as a slow-but-steady thing rather than a quick fix. If you want more info, search "carbon dosing" or "vinegar dosing" on this forum.
