hi i brought a second hand tank a few weeks ago. we filled it up before we got the heater and it was fine, a few days later we added the heater and it was still fine. 2 weeks later we added a filter and the fish, ever since there haas been water collecting at the back of the tank on the outside around the base. i have checked the wires and there not wet. we havew a back ground on the outside, this gets water between the glass and back ground. do i have a leak or is it condensation and what can i do about it?