If it's just plain chlorine bleach with no detergent additives it breaks down fairly quickly into little more than salt. I think the stuff you get at the store is often stabilized with sodium hydroxide (lye), the residual amount would increase alkalinity... Still not really toxic long term though. I disinfect tanks, buckets, nets and many things with bleach as needed, there is no residue after rinsing. If there was enough bleach left to cause harm, I am pretty sure you would have smelled it.
Really sounds more like there was more than bleach in that bucket. Some kind of laundry detergent or something else more than just good old bleach. That is harder to get rid of and breaking down the tank is the only way to be sure.
Rainbow cichlids are pretty tough fish, takes a lot to kill them. Sorry you lost yours, they are among the favorites of the cichlids I keep.