I second Red's advice, nitrate may be too low and CO2 is probably
insufficient. What macros do you dose? What are your KH, PH and NO3
levels? May be worth checking PO4 levels too. Chances are one of these
items is limiting plant growth therefore the extra flourish goes
towards feeding the algae. Not convinced on the light issue though, if
they are old tubes then they are putting out less light and probably
helping you avoid even worse problems. Changing the bulb before fixing
any nutrient imbalance may actually make things worse. Besides the
reduction in light output, I have never had algae problems due to
aging bulbs, just a slower growth rate.

Hope that helps
Giancarlo Podio