First on the light, this is likely the main problem. This is a 75g tank, so I assume 4 feet in length? And the light is a single tube 48 inches in length? Whatever the tube, this is really pushing it. And I note you have substrate plants which generally require more intense light. Are you prepared to get a new fixture, or a different tube? We can discuss.
Second, the nutrients. I would definitely not mess with dry ferts here, this is likely going to cause a lot more trouble. Light and nutrients (of which there are 17) must be in balance, at whatever level, in order for plants to manage. Increasing nutrients with low light is going to create an algae soup.
Stay with the Flourish Comprehensive, and use it once a week. In a 75g, this means 1 full teaspoon on the day following the water change. If you improve the light, going to twice weekly might be advisable. Either way, this is not going to use much Flourish. And to save tons of money, buy it online, and consider the larger size. With my 7 planted tanks, I go through about 1 jug (2 liters) a year, which costs me $30 [in the USA you will likely get it for less than this too], which is not bad for what it does.
On the GH, this is a problem too. However, you might manage with the Flourish Comp, plus Flourish Tabs for the larger plants like the Tiger Lotus and large swords. I use the tabs for these and it makes quite a difference. The Tabs last 3 months.
Byron.