I am currently treating my fish (including oscar) for ich after I saw they had it after a vacation. I am using Ich Guard 2 from Jungle (has formalin as active ingredient), increased temp to 30 degrees, and am using 1 tablespoon of aqarium salt per 5 gallons. I am doing about 40% water changes per day on the 90 gallon tank they are in. Ammonia is zero, nitrate about 20, and ph 7-7.5. I was wondering with salt in the water how long they can handle a nitrite level of 0.1-0.3? I was wondering because they still have clearly visible signs of ich and fish that showed no signs of ich because I can't see their gills, now have visible ich on fins and body parts. It doesn't seem to be going away so I need to continue treatment. There is an oscar, red devil, green terror, and sun catfish, all still juveniles. The seem to be doing good, but I am unsure of how long they can stand nitrites at a level of 0.1 to 0.3 like this. I am also unsure of how long it should be before visible signs of ich will start to dissapear as I have been treating for about 6 days now. (I have made sure not to ever let temp drop below 30 degrees even during water changes)

When it comes to raising the temperature of the water and medicating the tank, you should really any do one at a time. The warmer the water gets, the less oxygen there is in the water. This can also happen when medicating the tank, so it is probably best not to do both at the same time. Also, I would advise against doing a water change whilst medicating the tank, if you do that, you'll just be removing some of the medication and replacing it with fresh water. You should leave your tank temperature at 30°C for at least 10 days. Read the section on treating white spot, it's extremely good.

Nitrite is highly toxic, you certainly don't want nitrite levels staying above zero for too long. There is a medication/treatment called Prime, I think this detoxifies elements such as ammonia and nitrite, it could be worth trying. It's also worth noting that the salt can also be used to reduce the effects of nitrite in fish. From my experience, I do think quite a few different species of fish are quite resilient and can actually put up with a certain amount of ammonia and nitrite. I've had a few times when I've had both present in the water and I haven't lost fish. Having said this, you want to try and avoid the situation at all costs. Even though you might not see any signs of ill health, you don't know what's going on inside the fish. Dangerous toxins can damage the organs & immune system, which in turn can lead to ill health in the future.