wel you can have a decent amount of fish in that big of a tank you just need the right amount of filteration. another thing... did you recently buy any new fish? That is always another way you can get ich. To prevent this, dont put any new specimens in the same tank... set up a quarantine tank and treat them in that before putting them in your main tank. Also get the right temperment balance between your fish. if you have a overly dominant fish you might want to try and get rid of them or figure something else out. good luck man, hope al works out well for you without any losses.