You can simply buy a T-5 fixture and set it atop the tank above whatever spot was designed for light to shine through. In my case, I have tanks that I have covered with glass and just set the lamp above them. On another tank I have a tank lid made to place the manufacturer's T-8 fixtures above it and I simply placed a T-5 HO directly where the T-8 was designed to go. In the US I got mine here but I have no idea where to find them in the UK if you are there.