Flourish Excel is a carbon substitute, not a fertilizer.
Once one gets into plants needing moderate and higher light levels and ising additional co2, one needs to have proper lighting and proper fertilization. Failing to have these three things in proper balance will result in algae.
For example, I used to have a pressurized co2 added planted tank. it was a ton of work. I had a couple of algae outbreaks because Lights wore out or I let the co2 do so and I got bad algae. When away or ill and failing to care for the tank once or twice, I got algae. After about a decade I gave it up. Now I stay at the lower end of light levels. I use Excel, but for many years I also used phophate, nitrogen, potassium, and iron plus Tropica's Micronutirent and trace mix. I have a recipe which is supposed to duplicate this product, it uses dry ingredients. It does not work as well but if I use it 50/50 with the Tropica the plants stay happy. They will be happier with 100% Tropica, but the stuff is pricey.
I have recently gotten more lazy and more able to over-pay and I am messing around with the Tropica Specialized fertilizer:
Premium Fertiliser has all the required micro nutrients and is recommended for all plant aquariums. Specialised Fertiliser further has macro nutrients and is suitable for aquariums with many plants. Both are liquid fertilisers that are dispensed directly into the water on a regular basis.http://tropica.com/en/plant-care/
The Tropica site can teach you a ton about plants in tanks.