I've had a 25lr tank set up for a few months now, which contained one goldfish which I added when the tank was up and running healthily. I have some small plants and a snail, and everthing was going really great up until recently. I bought a white goldfish just over a week ago and added to it my tank, which already had one other goldfish. Soon after there was an ammonia spike, despite the fact that I keep up with regular water changes. My first goldfish died, my new goldfish seems to be very lively and not at all affected in a similar way. The old goldfish was a Black Moor, I'm fairly sure his death was a result of the ammonia etc as he was gasping and didn't seem to have any obvious physical abnormalities. I've increased water changes in the hope of sorting this asap. However toward the back of my new goldfish's body on one side a pink patch has appeared, and a small red mark in the tail, which was not present before. I am really concerned that it may be septicemia. Any help would be desperately appreciated, after losing the last fish I am horrified at the idea of losing this guy. I've tried to attach some photos but apologies they are pretty bad quality as he's incredibly lively and I couldn't focus the camera on him.