When my last light unit broke the manufacture told me it was due to condensation damage, created by having my air pump turned up too high! Does this sound correct?

Having lots of airbubbles from an airstone can cause water to get onto light units. However, moisture in light units cause them to fizzle and pop. You get moisture on the metal components and over time they either corrode until the metal no longer has proper contact, or the moisture allows electrical current to short circuit and melt something in the unit. If the unit didn't give you a tingly sensation when touching it, or didn't produce a horrible smelling smoke then it prbably wasn't moisture related.
If you have a coverglass on the tank that will prevent excess moisture getting onto the light unit.

thanks for excellent reply. think i wil invest in a cover of some sort