Nutrient deficiency in both of my planted tanks

[Eackone]

I have 2 heavily planted tanks with a lot of hygrophila polysperma in both of them, recently (for past month) they have been strugling a bit more, the lower leaves are falling off and full of small holes in all my tanks, I have a radican sword that doesnt seem to grow much since I added it in the tank too.
Both tanks have a soil substrate capped with sand

I dose the upper recommended dose of Flourish twice a week and flourish trace twice a week. I use flourish excell as my carbon source every 2 days.

10 gallon - Hygrophila polysperma, s.repens, various cryps, java ferns and dwarf lettuce
Ammonia 0
Nitrites 0
Nitrate 5

29 Gallon - Hygrophila polysperma, s.repens, various cryps, amazon sword, radican sword, dwarf lettuce
Ammonia 0
Nitrites 0
Nitrate 5

I've been reading its likely due to potassium deficiency from what I understand. IF thats the case, would I be able to increase how much flourish I dose or I would need to buy supplemental fertilizers?
I'll need to buy a potassium test kit i guess..

[Kalemh]

Im not sure if they make potassium test kits. I would recommend maybe buying flourish potassium. Fluorish has low amounts of potassium nitrogen and phosphorus. So if you think its potassium I would recommend flourish potassium or buying some dry fertillizers which is what I use. 

[Eackone]

Thanks for the suggestion.
Got a small bottle of Seachem K to try it out.. if I see improvement and confirm my diagnosis I might need to look into dry ferts