found this damselfly on top of my aquarium
No interactions present.
This is an immature male of one of the Blue-tailed Damselfly species. It closely resembles a male of the common Blue-tailed Damselfly (Ischnura elegans) which is widespread in Britain, including Herts - but it doesn't look quite right, and it may be a foreign species which has emerged from a larva in your aquarium (such as Ischnura senegalensis, which is a widespread species in Africa and Asia).
It will probably not be possible to identify the species from photographs like this (it would be difficult even if a detailed series of close ups of the relevant parts were taken). Have you released the damselfly outside or do you still have it captive somewhere? If you still have it, or are able to find and collect the shed larval skin which may be somewhere in, on, or near your aquarium it may be possible to get an identification (the shed skin will be very fragile so take care if you find it and try to collect it!). If there is no shed skin anywhere near your aquarium, then it seems most likely that it is a Common Blue-tailed Damselfly that has flown in through a window.
If you do still have the damselfly, or find the shed larval skin, and would like to try and get an identification please get in touch (my email address is at the bottom of my profile page if you click on my user name).
I am the county dragonfly recorder for Hertfordshire.
Thanks for the help guys, i totally love this site.
After seeing the insect I can confirm that it is a male Ischnura senegalensis (although this required examination with magnification and can't be confirmed from the photo, so I've left the likely ID as "Ischnura species.
Lat/Lng: 52.057256, 0.029896
OS grid ref: TL392418