I'm so frustrated I have been trying to set up my aquarium for the longest. Every time I plan on doing something with or for the aquarium there is somehow a set back. I am new to this but I have set up a 15 gallon tank. For a substrate I have used sand. I have put in the decorations and have everything where I feel it will be comfortable for my betta now the problem is when i stick my hand into the tank only the upper half of the tank is warm and once you get to the bottom on the tank it is extremely cold. The heater that I am using is a Top Fin 100 watt submersible. When I put the thermometer on one side it reads at 82 and which is near the heater and on the left side of the tank around 76 mind you that my heater is on 93 degrees. What am I doing wrong? I really hope this isn't a dumb question but is the water near the sand suppose to be cold?