My fish are sick :(

Question
Hi! I have a 100 gallon tank with a 12 inch Oscar, a 11 inch Leporinis named Leo and a pleco. I ve been talking to so many people about my fish and I thought this might be good to try.

So I know now that my fish are overstocked and that my filter needs to be upgraded. I am working on that and now have there perameters at a good level. they are feeling a little better now but i am still worried.

It started with twitchy behavior and lazyness. Oscar would be sitting at the top of the tank with his head like poking out of the water. He started to have flipover and became very unbalanced. Also I started to see reddness on Leo and he was scrapping himself on the things in the tank. I was getting told different things to do for him. The old owner told me to do a dose of mela fix and then called the next day saying that she thinks he has velvet and to get copper sufate. so I went to the fish store and got clout instead as the fish store guy recommended it. That day I did the clout and I think it was their worst day ever. the next day I did a 75% change and now today a 50% change. They do seem to be a little better. Oscar is swimming around now but still acting a little weak and unbalanced. Leo is still red but I think it is healing a little. Another thing I noticed was Its like I can see all of oscars red veins or somthing. Mostly in his fins. So now that there are no meds in the tank,,Im just not sure what to do. One person suggested it may be Septicemia? Should I treat them with antibiotics or should I just wait it out and keep doing water changes? Anything would help!!! Thank you so much and I hope you can understand my babaling.

Answer
Hi Lauren, it sounds like your nitrite levels got high. This happened in my tank before and threw everything off. A lot of my fish died and I couldn't understand why. It also sounds like your fish could have Septicemia and the only way to treat this is medicated foods which you can purchase at your local pet store and melafix which it sounds like you already have. Also they could have ick if they were rubbing their bodies on the rocks especially if they have white spots so look for that. If they do have ick you can purchase a medicine at your pet store and fix it. As for the nitrite I would do a daily 10 water change for about 3 days and then do a weekly 10 water change for about a month. Then do a 20 water change every two weeks after that. This will hopefully get you back on track. Also add a little aquarium salt after every water change and your tank will be much healthier! Good luck!