One at each end gives you a safety net. If a heater fails, you will still have something to heat the water until it can be replaced. It will also heat the water more evenly but in a 90 that should not be a problem since it is only 4 ft long. My 6 ft 120 I used a 150 and a 200 with one at each end. the filter actually moves water around enough that I rarely see them both on. whichever one comes on first seems to do the trick. Since I change settings somewhat with the season, they are never set at quite the same amount so one ends up doing all the work.