pH isn't causing your plants to struggle. More than likely it's the "small quantities" of Tropica Specialised that you're adding. You're using CO2 with what looks like a decent amount of light along with an inert substrate. Tropica Specialized, although it's an all in one fertilizer, is pretty lean. If you're not using the amount that's recommended on the bottle than I would try that amount for a couple of weeks at least. If you're using the recommended amount then I would double it. That is all assuming that your CO2 is adequate. If it isn't then your plants are going to struggle no matter how much you fertilize them.
So, get your CO2 in line, fertilize adequately for the amount of light and plant mass that you have.
More info would really be helpful though such as exactly how much Tropica Specialized your adding. How much CO2 your adding based on your pH with the addition of CO2 versus your degassed pH and some idea of how much light you have going.