I have 5 turbos (2 larger, 3 medium size), in a 120 gallon and I find myself constantly having to feed them nori on my algae grid. I believe 1 of my larger turbo snails produces more poop than 1 of my 3" yellow tang. I would not recommend more than 1 medium size turbo for your tank for the time being. If algae is becoming a problem better to add one later but I bet that 1 turbo would keep your tank clean as well as become fat and healthy. Unlike fish they can't go and scavenge once algae is out or minimal and with 5 they are certainly starving themselves out. In my previous 120g tank I had only 1 large turbo snail it grew to 3" and was my only clean up crew member and it did its job. When the turbos have enough food they will eat at night and sleep during the day. If they are starved they will look for food during the day; when this happens I know its time to supplement feed. If they are sleeping with tons of poop around them I know Im good.

In your case If they are at a point of no return it may be hard but try nori on a grid or rubber band it to a rock and put it right infront of the turbo. If it eats the food it should recover. I don't believe its your ammonia but it may be a factor so definitely take care of that.