Thanks for the responses :)
I noticed after changing his water last night that it took overnight for the temp to go back up to 26, so thats a good thing right? That its gradually increased to the right temp and not suddenly?
Gosh who knows i just had the biggest freak out!
The brand is aqua one and its 25w.
I had a 15w one that was causing temp swings.
I also purchased a new thermometer just to check and it was reading the same temp as my current one.
PS he HATES the new thermometre!! He flared up at it!!! Needless to say its not in there anymore... fussy little thing!