Pleco Death

I came home from work to find my pleco dead in the tank. Heidi was normal up until this point; giddy at night time, hid away during day time. didn't appear to be getting any abuse from the other fish (1 dwarf fire tail gourimi, 7 neon tetras, 1 dwarf Cichlid). The water temperature has been constant at 26 celcius to the best of my knowledge, readings up until this even were 60 (GH), 80(KH), 6.5 (pH), 0 (no2), 0 (no3). I took a reading after i fished heidi out of the tank the GH was at 120 and the KH was at 120. I did a 30%water change 3 days ago and all readings after this were consistanst with previous readings. The only differences I can think of are the following
4 new fish were introduced 4 days ago, 2 neon tetras & 2 dwarf Cichlid one of which the gourimi harrassed to death (the female, the male has survived. We have also started using new fish food 'nutrafin max' and sinking tablets for the now dead pleco 'new life spectrum h2o stable wafers'. Along with this we have introduced two fake plants and one ornament. The tank size is 59L and we are using an aqua one 101F pump with the duck bill attachment to provide air into the tank as well as filtration. Following the sad discovery I have done a 50% water change and and added the same treatments as I have been previously. the tank is only 3 weeks old, and the pleco was one of the first fish we got.

Anyone any ideas as to why she died and what i can do to prevent further deaths.

I'm very sorry for your loss. I absolutely love plecos, one of my favorites.

If your tank is only 3 weeks old, sounds like an ammonia spike to me. You didn't mention if you tested ammonia I believe. With that many fish in such a new tank, I'd say it's a likely culprit. Was the tank fully cycled before adding fish?