But if it's ammonia you risk doing permanent gill damage to the survivors or worse yet losing them all.

Amazon has a good bargain on API test kits.

Your best bet if you think it's ammonia is to run in fresh water ASAP and flush out the old water. That can buy your time to reduce the fish population.

BTW nitrates can also be detrimental to trout but other species tolerate them well.

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.