I have learned the hard way, that if you use a Hydor in-line it is a good idea to put a check valve below the heater to prevent the line from draining during a power outage. If the drains dry of water, the heater easily burns up.
Someone posted here on MFK about building a DIY heater using the element from a standard home water heater, and using a Ranco(sp?) controller from a hardware store, I don't remember where the thread is, but it seems like a great idea to me.
Might be really good combined using Mudbutts method.