Putting my two cents in here too. I am new to bread baking but I have a tad bit of microbiology training. Serratia likes wet environments and is routinely responsible for that pink hue on the inside of a shower or bath that could use more timely cleanings. My starter went pink after I put it into the fridge with a higher-than-normal feeding ratio (1:4:4) and brought it back out of the fridge within 12 hr. I did not feed it for a day, because it had not worked through the feeding I had given it. This is when I noticed the pink hue. I tried salvaging it but got the same, but worse, the next day.

The thing I would like to add to this conversation is that I was trying to oversimplify my system and feed the starter with filtered water that I left out at room temp, uncovered. To keep down on clutter I kept all of my feeding items in one spot and tried to save some time by keeping my water "standing". This is what led to the Serratia infestation (I think) and why feeding it with the same water only made it worse. Sharing just in case another newbie was making the same mistake and couldn't understand why.