I am getting conflicting tempreture readings which is causing stress (to me) as well as my fish i'm sure. I have a small grey digital battery operated thermometer which has a white lead prob. I also have one on those cheaper glass floating thermometers. They differ by 4 degrees. What should I do. Spend money on good water heaters which have their own inbuilt thermometers or buy more expencive thermometers with supposidly cheaper heaters. My system is 200litres so should I have 2 smaller heaters or just 1 large? I realise the more one spends the better the product but we all know about budgets so maybe someone has a good working combination.