Good responses from other members; a couple of things stand out at me in your original post, so I'll tie all this together if I can.
First, the light. These tubes I know, they are in my view not the best for planted tanks, being high in the blue which as someone mentioned promotes algae at the expense of better plant growth. Changing to full spectrum (with a kelvin around 6500K) will help in several ways.
Second, the Kent ferts. Pro-Plant includes nitrates, and I have read elsewhere that this can encourage algae. I tried to find the ingredients online and couldn't, but going from memory when I looked at this in the fish store it is missing a few nutrients. It also takes a lot, so you go through it fast.
As for the iron and manganese, this could be causing trouble too, at 2-3 times a week. Iron and manganese are micro-nutrients, along with copper, zinc and some others. If these are not balanced, plants will not improve and again when that happens algae uses the light and increases. Iron and manganese are also heavy metals, which is too great a supply can be toxic to fish, plants and bacteria. These mineral nutrients need some control.
My suggestion would be to consider getting Seachem's Flourish Comprehensive Supplement for the Planted Aquarium
. All essential nutrients are included (save oxygen, carbon and hydrogen which obviously occur naturally in the aquarium) so you only need the one product. And you use very little of it, 1/2 teaspoon treats 30 gallons and only once or possibly twice a week. Long-term it is less costly.
Normally, reducing ferts (nutrients) is not a solution to solving algae problems because algae uses the light when the plants can't due to insufficient nutrients in balance. But in this case I would reduce the ferts, esp the iron and manganese. And replace the tubes which will make a big difference. If you change the lights, you can continue the Plant-Gro and iron/manganese but less of it. This sentence comes from Kent's website on this product: Healthy plants also require proper spectrum lighting and intensity to maintain color and for growth. They are correct. I believe this will help you control the algae. Byron.