Large anemone put out lots of waste, plus it will sting corals that it touch. I think you just got too much bioload on your tank with the anemone, rather than it is putting out any toxin or anything else.
There are many large anemones in tanks full or SPS or LPS without problem as long as you can keep the corals away from the anemone and make sure it doesn't put out too much waste/ or having enough capacity to process them.
You got to feed it quite a lot to have it grow from 6 inches to 2 feet in 1 year (in a 75 g tank). After you remove it, no more wast produced, and your tank recovered. I am sure if you was measuring ammonia and nitrite, it got to start to increase near the end right before you remove the anemone