I haven't done any water changes since i've had my tank
How long have you had your tank?
I do a 30% water change every week, if anything does break out (which is rare) I do a 40% water change (maybe a little less if Ive done one in the last couple of days) treat the tank then leave it at least 4 days, then change another £30% of the water, then dont treat the tank for another 3 days......if things havent improved I treat it fully again.
Rather than not treating it for that 3 days you could treat it again after the initial 4 days with half the recomended dose as i know some people do but I personally dont as over treating a tank can cause future problems.