I mean like if theres like a visible inch of water missing, reason being that neither of my bettas like the filter current and with the water a little lower its more of a tiny waterfall than water being pushed around. Its nice to feel that half the tank isnt being wasted since they actually go to the left side often now that theres no current.
Mine is the same way in my divided 5 gallon tank. The current pushed my betta over! It was sad so I have at least an inch and a half that isn't filled and the filter is just a nice trickle. My betta now has a huge bubblenest! :)
Oh wow i totally forgot about covering the heater, i decided to fill up my 20g cuase the heater wasnt covered and i have a cut up pop bottle to break the current in that tank anyway. Are all heaters fully submersable? My heater in my 15g i know is so i left that one a little low since i dont have the cover to break the current.