It is difficult to tell exactly what nutrient is missing at this stage. The species of plants in the tank are not extremely good indicator plants. Also, whatever the deficiency is it is very early on and not very clear yet.
I'd say it is probably nitrogen if anything since some of the bacopa and anubias leaves are deteriorating from the tips of old leaves and nitrogen is usually the most needed nutrient out of the macros/micros.
You really need to start dosing all the nutrients if you want to keep plants. You'll likely go from deficiency to deficiency if you just add one at a time. Flourish excel is not a nutrient.
You can start out by adding low amounts of each nutrient to your tank on a regular basis and go from there.