Okay you probably over dosed the conditioner, nitrates aren't going to kill your fish, nitrates do affect the immune system so they have trouble fighting off illnesses, for future reference 50% weekly is over kill, you added 32 ml of conditioner which yes you can over dose , he probably had something else wrong, what type filter do you have, what type substrate, what size tank, also for future reference don't fret over nitrates unless above in my personal opinion 60 as water changes and filter maintenence will lower them, BTW find a new lfs
, water conditioner really doesn't do anything with nitrates... Red devils need a huge tank as they get very big, and they also prefer heavy current.
30% water charges every week, if filtration isn't up to par upgrade, when doing water changes do a deep gravel vac to remove waste, condition the water with prime water conditioner as that's the only one I would recommend, I see no one asked tank size, filter, what ammonia and nitrite were, if there was any outwards signs of illnesses, a lot of things were missed on this topic so I'll ask you even though he's already gone
What were your whole readings
Filtration (brand/type of filter)
Were there any signs of illness
If you want to do 50% weekly break it up into 2 25% changes
How do you do your pwc
, with a bucket and siphon or a python that you hook up to your faucet.