I believe your tank has slime in it, and the ionic component of the bleach helps conduct electricity through the wet slime. You didn't use enough bleach to kill and remove the slime.
I sanitize quarterly, per manufacturer recommendation. I don't quite remember the ratio, but I think it's 1/4 cup per 15 gallons of tank capacity. The instructions say to add 1/4 cup to 1 gallon of water to fill through siphon and water pump. Repeat as necessary, to tank capacity multiplier. I have a 100 gallon fresh tank, so I use 1/4 cup x 7. Then, fill the tank till it comes out the overflow. Then run it through all plumbing. Don't forget washer, toilets, icemaker, and outside taps. I have a cap directly on my fresh tank, so I add about 2 cups directly into the fresh tank. I need to remember to run some through the siphon inlet, when I use this method..
Leave the solution in for at least 4 hours (I do overnight). I also don't sanitize my hot water heater. I isolate it, then siphon rust and lime with a Tygon hose from Home Depot. Also remove and jump any filters before sanitizing, bleach will be removed and load up the filter IX bed.
Do a good job of rinsing. Hope this helps.
We travel with 2 Shelties and one '1 of a kind' from the shelter. They love it!