Firstly ill ask, did you do any ammonia tests whilst this was occuring, also your nitrate shouldnt be 0 normally it should be at 12.5. My guess is that adding the new fish may have triggered the tank to cycle, and if this happened the huge waterchange might not have been such a good idea.
soz to hear about it, no-one likes losing fish.
soz to hear about it, no-one likes losing fish.
Holy crap that's specific!
A tank setup for 4 months will likely only have enough BB for whats in the tank, and maybe a little more.