Well, if you have done 2 water changes in 7 months, that's probably a problem. Water changes should be done once every week or two, 10 to 20% of your tank's volume at a time (unless you're me, of course. And then you totally neglect your tank and never ever ever do water changes!!! ;)).
i think i would neglect mine if i had to change 40 to 50 gallons or more for a water change