- a heater is best but I find that with vases and bowls with the curved glass, most heaters won't fit. I would think something like this would work. I have never used it so I can't say how well they work. Also a theometer is needed to keep an eye on the temp. I don't think this has an auto shut off which means it will stay on even if the water gets too hot.
Also, I know people have already mentioned the water change thing already but its very important to do the weekly changes to remove the ammonia. When you do the 100%, you also need to be cleaning out the gravel to remove any poop and debris that has fallen into the gravel. if not, it will pollute the water