tank temperature fluctuations

[bobblue]

I got my 20 gallon tank second hand and with it came a thermostat......it seem rather old and although it worked I decided to invest in a new one thinking this was a safer approach. I only paid about 30 dollars for it but it's been in my tank for 2 and a half months and up until now the temperature has been fairly constant at the set value of 77F. Yesterday I noticed that it was pushing 80F. It was an unusually warm day and I thought this might have something to do with it and over a few hours it would right it's self. Never the less I turned it down slightly. When I came home from work a good 12 hours later, I saw that the temperature was pushing 82F!!...as I am writing this, I put back the original termostat and which I set to 77F....it's too early to tell whats going to happen........am I wrong in assuming that a termostat should regulate it's self better then what I just experienced. I'm hoping that the one I bought was just too cheap and simply malfunctioned and this is not what would normally happen even if there is a significant increase in outside temperatures. Any thoughts???

thanks

[glarior]

It could be a faulty heater. If you can return it for a new one. Which one did you get? I have had faulty heaters before and they killed my fish!

Also, depending your house temperature the water will adjust up and down through out the day. Since its only 20g the temperature will adjust quicker than if you had 75g.

[KatzeSlaugen]

my heater does the same thing sometimes. it will work fine for a few days and then it will just stop turning on all together. ive decided its a problem with the springs that are used as the elements because when i shake my heater it will turn on. wish it happened before the 30day return policy because i dont have the box, know if there even is a warranty, or have money for a new heater at the moment