Help! Multiple Death's

Hey I take care of someones fish tank every 2 weeks. I went there and did a 25% waterchange. Tested basic levels PH= 8.0 Nitrate = 10 Nitrite = 0 Ammonia = 0. A few days later I get a call telling me 4 of there 5 fish died. They told me that there Yellow Dogface Puffer was acting strange and not eating and started floating around the next day he died. Soon after a ****** Trigger and 2 Tomato Clown's died. When I got there I tested the water and all the water levels were WAY outa wack. PH = 7.4 Nitrate = 100 Nitrite & Ammonia I dont remember the exact PPM but they were half way up the test chart. This is a well established tank.

My question is does the Puffer release Toxins when it dies or something? It seems like after the puffer died then the other fish got sick. This tank never had a high waste level of any kind. There also was a fair amount of Red Slime Algae and Brown Hair Algae all over the Substraite( They messed with there timer and screwed it up and had the light on for 24HRS a day for a few days

Since this happened I did a 50% water change and 2 days later did another 50% and the Levels have all come down. PH = 8.0 Nitrites = 0 Ammonia = 0 and Nitrates = 40 (Little harder to get this down as they have 10PPM out of the tap)

Any other thoughts? Thanx in advance!

So the Puffer does release Toxins.. **** that sucks. Amazing enough 1 Tomato Clown did make it and is still running strong. The Puffer was in the tank no more then 12 Hrs after he died. They left the office at 8PM and he was alive and when they came back the next day 9AM the Puffer was dead and all other fish started looking sick. They died soon after that.

I just want to make sure that the Puffer dying is the root of the problem not something else...