I spent many year, and still do to some extend, in engineering field dealing with temp controls. For what is worth - this is my set-up on my 80 gallon reef/fish tank with a 20 gallon refugum.
I have two Rena Cal Top Light Exels; 300 W & 150W. The 300 W is located behind the overflow shield in the main tank, and the 150W in the refugium.
I heard so many stories about deep fried fish b/c of faulty heaters. So, not considering the cost of a digital temp controller, my set up is very reliable and precise. I have a programmerable temperature controller set at 77.0'F and the heaters' built-in thermostats set at about 79'F. My digital controller (typical dig controllers cost about $100 - $200 retail) controls both heaters and maintains tank temp to within 0.1'F of set temp. Without going in too much detail - heaters stay on and off about 1 second interval when controlling, so the heaters never get hot. In the event the digital controller fails in closed output mode, the heater's built in thermostat will control the heaters at its set temp - so over heating is slim, and the heater will last indefinately.
Down side of this set up is, if the digital controller failed in the open circuit position, the heaters will not come on. To remedy this, I could put a third heater or take the one of the two out the digital controller loop, and set it at a few degrees higher than the digital controller.
Looking at the heater design, I suspect the most of failures come from built-in thermostats. These built-in thermostats use some sort of mechanism that expands and contracts as the temp around the heaters change and open and close a heater contact. Also, I suspect these heater thermostats are effected by any physical vibration; high water flow around it, or fish bumping it could change the set point. So, securely mounting of these heaters might be important.
As for in-line heaters, since it only works if their is water flow, it adds another potential weakness in the heating system. Also possible leak problems and the high cost. I'm sure they designed it so that it would not reduce pump rating but you are dealing with added fittings which would reduce pump capacity. So, I am thumbs down on this set up.
Rick
