Use a normal boggo standard Thermometer. Or, if you feel flush, one of the proper digital ones. Nothing inbetween. I would use an electronic thermostatically controlled heater too.

Don't have the light on in the tank for longer than about 8 hours a day too.

Then get proper test kits for Ammonia, Nitrite and Nitrate. The tablet ones are easiest, but I would still get the ones where you add drops to a sample of water in a test-tube, they aren't that hard to use.