I have a 10 gallon tank that’s been fine for a while month I have 1 betta 4 bottom feeders and had 3 guppies. Today one guppy died with part of its tail missing, the others are fine though and suddenly there have been dark brown/black spots that don’t move near the tank and I did a water change around 25% 3-4 weeks ago to remove the algae and nothing will leave it keeps coming back. What should I do? I’m thinking of removing all the fish into a temporary tank and cleaning the whole thing out but I’m not sure I have no plants in it currently either!