Underpowered Heater And Thermometer

I have brought 2 thermometers and installed them today and I have a couple of questions

1. How long should I wait before I look at the temperature on the thermometer and take it as accurate.
2. If a heater is not heating to the right temperature which I think the new one isnt what should I do

From Tom

You don't say what type of thermometers you have. Having said that it should register the temp. In under 2min. Don't forget if you have the alcohol or stick on type they are only a guide.
If you suspect a heater is faulty replace it. You don't want a tank of boiled fish. If it stops working and the temp. drops this is not so bad as long as you check the temp. every time you feed the fish this gives you time to take action and replace the heater. I always keep a spare heater for emergencies

I have a digital thermometer like the one in the attached picture. It is set at 25c but at the minute the temp I'm the tank is 22 can I increase the temp On the heater to 28 in the hope that I will be at 25 and is it alright to bump the temp up 3c in one go. I do have a spare but it's a 50w and this one is a 100w so the 50w is no good from Tom

I have found a lot of heaters recently do not heat the tanks to the temperature you set on the dial, its really annoying . If both thermometers are reading 22c and you want it at 25 then yes it is ok to turn up your heater it will take a while for it to heat all the water to temperature, its not instant. Just monitor the temperature throughout the day if it does go to say 26 then turn your heater back down again.
 
Dont forget though room temperature can also have an affect on the tank temperature, a colder room would need the heater set slightly higher than a warm room. If you are using a 100w heater on your 110L tank it maybe struggling, go for a 150 or 200w heater, the approximate rule is 1w of heat per litre of water or if its possible add the 50w heater to the tank with the 100w.
 
Hope this helps

In my 110 litre tank I have a 150 or 200w heater and it heats it perfectly I set it to 24c and it's 24.2c so that one is fine it in my 90 litre with a 100w heater that it's 22 when I set it to 25. Also I only have one thermometer in each tank and I put it as far away from the heaters as possible to get an accurate temperature of the tanks water. It's not possible to add the 50w with the 100w as there are no more plug sockets available. Also I will turn up the heater to 28c in the hope that it raises the temperature to 25c from Tom

I turned my heater up to 28c over 2 hours ago and so far the temperature has raised 0.7c is it normal for the water to take that long to heat up or do you think there could be something wrong with the heater. What do people think to the delta therm heaters by interpet. I have one downstairs and it is set at the 24c and it's between 24c and 24.2c from Tom

Water has a very high specific heat, so YES it can take a while.

Ok thank you very much I will keep checking to make sure it's rising but I must say I'm not very happy that there scale is out by 3c as that is a lot and what do you think of the delta therm heaters have you had any experience with them from Tom

Also will the temperature of the water and in the tank be affected by me having the lights on as the water in the downstairs tank is now at 24.9 but the lights have been on for 3 hours and how accurate are the digital thermometers from Tom

Lights do raise the temperature very slightly depending on the lights.  Thankfully your heater is under not over, my worst one was a 300w heater set at 22c and it heated the tank to 28c. I would give it 24 hours to see what the reading is, dont forget though it may drop a little over night unless its in a heated room.