[MatPat]

You do have a decent amount of light over the tank and Water Sprite and Hygro (what type do you have?) are fast growers and therefore big consumers of nutrients. If you want to keep things simple, you may be able to get by simply by increasing your Flourish dosing. I assume you are using Flourish Comprehensive? Contrary to the directions in the flourish link above, I add about a capful to my 10g low tech tank each week to every other week when I remember

As far as knowing what "healthy" plants should look like, you can see pics of healthy plants in the PlantFinder

 

[MatPat]

Oh man, I totally missed the part in your post that you are using CO2 . In my opinion you definitely need to add more in the way of fertilizers...the CO2 will increase plant growth quite a bit even with 2wpg.

You can go the Seachem route for fertilizers but you will need to add more than they suggest. I can't be of much help with quantities since I've never used Seachem's line of fertilizers. However, you can read this thread for more info and a handy spreadsheet for using Seachem's ferts.

Should you decide to go the dry fert routine, EI is the easiest method to learn when you are first starting out with fertilizing. You should go with the amounts listed for a 10-20g tank. You can purchase a year's supply of dry stuff for about the same cost as a month's worth of Seachem's products.