try to buy submersible just because if your heater ever does fall into the tank completely, you know you'll be safe from any mishaps. when it comes down to titanium or glass, you should be fine with either. like grosse said, the via aqua titanium heaters have temp. adjustment controls seperate from the heating tube and they can withstand aggressive fish. if you have a glass heater, buy the heater guards made for hagen tronic heaters. they will fit most glass heaters, i personally use it and have no complaints.