I got the Top Fin divider at Petsmart to divide my 10 gallon about a week and a half ago, maybe longer. Everything was fine until I did my last water change on Wednesday night. I went to check on the guys and noticed that Sprinkles looked eerily like Blue. Well, it turns out that they switched tank sides. There was a just big enough gap along the tank divider and aquarium side that someone got to the other side. Blue is much larger than Sprinkles, and Sprinkles already had a traumatic week(cat tried to sit with fish while I changed the water) and lost some fin, but he was fine. Now he's all scraggly looking and Blue is missing a large chunk out of his fin. I am so glad that this didn't happen while I was gone this week.
I put Sprinkles back in his side of the tank after putting fake plants a an old tank lid back there to keep it shut off hopefully for now, and put Blue in a 3 gallon I had upgraded from. Blue is still agitated this morning and won't eat, and Sprinkles is hiding in his floating anacharis eating everything. I think it would be best to keep them in separate tanks until Blue stops flaring constantly and hopefully fix the divider.