Sorry long post here. But needing advice. I am new to aquarium life and had just added two Cory cat fish to my tank. Last week I came home and noticed one had died overnight then this morning my second one was in obvious distress and quickly died after I removed him to put him in a seperate tank from the others. Both were bought together from the same tank in the store. I'm thinking red spot disease?? As looking back they both had a red mark on them looking like internal bleeding that I only noticed right before they died. Need to know how to treat my other fish to prevent them from getting sick and the best way to clean my tank. I test the water weekly and have never had any problems with any of my levels.