Where the heck do you live that its only 17degC in your room!? lol I would say thats the problem there, 17 is really cold! I mean... its pretty much summer now. The ambient temperature in my house is about 26-30 and I live in the mountains in S/E QLD.. its considered fairly cool here, we are practically in a valley surrounded by rainforest. With aircon on the temp drops to 24-25deg. Those ceramic heat emitters have a very small range before the heat disipates in a mesh top enclosure. I would suggest that if your ambient room temp is so low you probably wont be able to use a mesh top lid for an essentially tropical species. There are a couple of things you could try... a heat cord may work to keep an area warmer than an emitter would... as will a spot light. But I think the most important thing would be to get the top of the mesh closed in. What is the ambient temp in the tank? If its 17 at this time of year then thats a bit cold. In your situation... I would get myself down to bunnings and build a box to put over the top of the tank that will fit the light fitting inside to keep heat in but has a door in it to vent a bit of heat if needed.