I have a 24x18x18 and am using an UTH with sand substrate. From what i have read, that should be enough for the whole tank, but other than the sand above the UTH, the whole tank is still 70 degrees, when it should be closer to 85. It has been about 3 hours, should i give it more time or go get a heat lamp? also, would 1 suffice or would i have to get 1 for each side?