Here's my two cents worth.... I've tried numerous methods to regulate heat and everyone one who has posted in this thread makes very valid points. What i've found is for MY racks I run hydrofarm single prob thermostats. but even with all of my racks being exactly the same set up (same amount of flexwatt per tub same double layer of news print for substrate same exact brand of tubs and size) there is still a 1 or 2 degree temp difference between my bottom and top tub (5 high racks double stacked so 10 per stack) heat rises. also instead of paying $150 and up for multi probe stats I opted for $30 hydrofarm stats and run one for every two racks however one thermostat runs the two racks on the top and the other runs the two racks on the bottom and I rarely find anything more than one degree differance in temps from top to bottom since I switched them over this way. Jmo if you are looking for a less expensive multi probe unit go with less expensive single probes and just by more of them. lol $120 for 4 probes and each one can be programmed for different temps if you are keeping different species. If anyone has seen this before and has seen a drawback to it please let me know also so I can correct it because i've only been running these for about 2 months now. seems to be working well tho