Why did you want the temperature to be higher than 26c?
The vast majority of tropical fish are very happy being kept at 24c and many will actually show signs of distress when kept above 27c. Fish such as loaches and barbs and other riverine species which come from large rivers which are cooled by the fast current especially prefer a slightly cooler aquarium. Remember that the warmer the water the less oxygen it can carry.
TF keeper is right about why the tank is warmer in the evenings. Water will warm to the ambient temperature of the room so in the evening when you have been cooking and may have had the central heating on the house will be quite warm and then chills back down over night as you sleep. This is not a completely bad thing as it is only mimicking what happens in nature, day and night temperatures can vary hugely in even in tropical climates.
