I have a 90 gallon tank which I had to leave for a week when I went on vacation. A relative fed the fish and without water changes the nitrates did manage to get to 40ppm. Ammonia is 0, nitrite is 0, and again nitrate is 40ppm. Sadly when I came back I discovered ich on 3 of the fish. The tank has a red devil, a tiger oscar, a green terror (or gold saum can't tell yet but they are covered in ich), as well as a sun catfish and a blue crayfish. Every fish in the tank including the crayfish is about 4" long. I am not sure how to treat for ich in this situation because I cannot really remove the crayfish and catfish and I know many medications kill or harm these two, as well, they may be sensitive to salt. Any help would be appreciated because I would like to start treating the fish. Thankyou.

Welcome to ofl

The first thing I would try is turning the temperature up to around 30°C and leaving it like that for around five days. the problem is, when you see the white cysts, they are immune to most medications. Turning the temperature up accelerates the parasites lifecycle. Definitely treat the tank to kill any free living parasites. If you look under helpful articles, there is a very good article about treating ICK, it's a very long read but it should give you some really good information. But try turning the temperature up for a few days, hopefully this should do the job

So I should just heat it to 30 degrees without using mediaction or should I treat at the same time?

First of all, raise the water temperature to 30°C and leave it like that for 10 days (sorry I made a mistake saying five days)

It is very important not to medicate the tank during this period. The problem with raising the water temperature is warm water tends to contain less oxygen. Medication can also cause oxygen depletion, so you mustn't do the two things at the same time. Do this for 10 days, if the white spots disappear, don't turn the water temperature down, keep it at 30°C.

The life cycle of white spot decreases with higher temperatures so it has less time to get to a host at higher temperatures. At 30oC it has such a sort life cycle that it will die out.

Turn the temperature up to 30oC. After ten days all symptoms should be gone. Leave the temperature at 30oC for another 6-10 days. You should now be symptom free for this period , if so return you tank temperature to what it was (bout 25oC), if you keep symptoms then leave the temperature at 30oC until you see no symptoms for 6-10 days

When I 1st started keeping oscars over 5yrs ago I got a few cases of white spot, I am now sure this was cos I didnt heat the water when I made water changes. As soon as I started to heat the new water, I never got another case, and all my fish have been well for around four and half yrs or longer now. I havent changed anything else and still do everything the same way so the tempery drop in water temp must have an effect??