Not to burst your bubble but i will put money on the lawnmower blenny, especially if ur diamond goby's are dying. i tried to put a lawnmower in with my diamond and in a split second BAM he was on the goby like stink on **** lol. also only 1 diamond per tank and same goes for lawnmowers.
i have heard of others having success but i personally have never been able to keep more than 1 of either the lawnmower or the diamond goby and its one or the other.