I bought a SunSun 100 watt submersible heater for my tank. I put it in the tank yesterday afternoon and set it to 78f. I checked the tank temp this morning and it's only at 75f. I noticed the heater will only turn on for about 5 minutes after I adjust the temp then not turn on again. The directions were not good as it's translated from Chinese and don't really give you any information of how to use it or where to position it. I currently have it next to my filter intake on a slight angle ( otherwise it wouldn't be fully under water. I can see it's working when it's on for those 5 minutes..Has anyone used this before, have any tips or info about it?