Having a spot of trouble, I'm new to blender, I've added mirrors to meshes fine no problems, but I'm now trying to mirror a mesh and its not mirroring where I want it to be, I need the mesh to be in the same location on the other side of the tank
The object that you have set as the Mirror Object ('Cylinder') on the object you want to mirror ('Cylinder.010') is not at the centre of your tank, therefore, neither is the mirroring.
If you still want to use a Mirror Object then choose a different object to use as your Mirror Object (one that has it's origin at the centre of the tank).
For example, here I change the Mirror Object to be 'Cube.004' and the mirroring is correct:
This is because of the location of the mesh you are mirroring, you need to place it as shown below and it's mirror location will change as well:
You need to set the point of origin as well. To move the origin, typically one would use shift + S -> cursor to selected to snap the 3d cursor to a desired location and then you can move the origin with shortcut Alt +Ctrl +Shift +C Or by pressing "origin" on the (T)ool panel.
Added gif showing all the steps: