As Raz said - insulate the tanks and check your temps throughout the day and evening...and if you happen to get up during the night for any reason, go ahead and pop in then too
Is it your house or a rental? I ask as ceiling insulation is a big thing in keeping your Fishroom warm. Hot air rises and if there's no insulation in the roof, it will escape. Drafts under the door in or through the sides of the windows also needs to be addressed. Glass is a huge heat conductor, so as your room heats up, windows will let it straight out (or in during summer for the reverse problem of over heating). Shades a good, shutters a re better. At a minimum, cut a piece of polystyrene to insulate the window and tape it to block off drafts if need be (and you're not fussed about how it looks).
Then insulate the tanks. Wrapping towels around and over the top of your tanks will quickly let you know if this will fix your fluctuating temp issues. If the towels work, then move to stick polystyrene to your tanks.