Size of tank? Filtration? Water parameters (nitrate, nitrite, ammonia, temp, ph, etc)?
I woke up to him this morning looking like this. https://ibb.co/bA9c4n
He was still breathing when I came home in the evening, so we made the decision to euthanize him in a bucket of ice water. He didn't even put up a fight when we tried to remove him from the tank.
I know this is a hard decision for any pet owner to make and that some wouldn't make it, but I couldn't stand to see him suffer any longer. He was such a lively piranha in his prime, so seeing this transition was difficult.
My other red belly is starting to develop the same white spots as the other did. My main focus right now is to try to not let the same thing happen to him. So we vacuumed the rocks, drained half the water out of the tank, and refilled it.
The tank is 27 gallons and the filtration is a Fluval. I don't have a water testing kit readily available at this moment.