Well, the fluval stratum is great, but bear in mind where your anubias lives. It isn't in the substrate, it is up on driftwood.
Because of this the nutrients in the substrate aren't available to the anubias. However, it will readily take in fertilizers added to the water column. Those water column fertlizers will also help replenish the stratum, keeping it rich for your substrate-growers too.
Looking at the pictures, there's definitely some potassium defficiency going on, possibly a bit of phosphorus.
You have two basic approaches here:
1) go with pre-made bottled fertilizers from folks like API and/or seachem. With a 5 gallon low-tech tank, these should be fine and won't get terribly expensive (trying to use them on a 75 gallon high-tech tank gets very expensive).
If you go this path I would suggest starting with:
API leaf zone (potassium + iron) and Seachem phosporus (phosphorus, obviously).
Seachem Flourish complete (Iron+micros), Seachem potassium, and Seachem phosphorus.
2) the other option is to go with DIY dry fertilizers. This is by far the most cost effective way, but it involves learning a few things and measuring and mixing your own solutions in water. It isn't terribly hard. That said, it would probably take your tank 30 years to consume one starter pack order. nilocg sells them in the for sale area here, and also sells a few kits for making your own bottle-mixes, although they would need to be diluted down some for a 5 gallon tank...
green leaf aquariums is a good normal web-store vendor many here use (I have not):