The foe that is flexwatt

[Desert]

in my rack of 10 tubs i use a herpstat ND set at 88F and place the probe under the second tub from the floor. the lower hot spots are always a few degrees cooler than in the higher tubs. i have hot spots range from 89-95 from the bottom to the top. it kinda sucks but i guess heat will always rise...

Heat will move in any direction; up, down, sideways, whatever. Heated air will rise. As do heated liquids. But heat itself moves in any direction.

As for this thread's opening post, I suspect the heat strip is working just fine. What most likely is being observed are differences in environmental conditions along that heat strip's length that affect the conduction of heat from it.

The remedy for this, (if a remedy is necessary) is to make the physical and thermal conditions as uniform as possible in the areas along that heat strip where temperature will be measured.