First of all, Coco Coir/Fiber is a hydroponic media and should be fed as such. The appropriate pH for coco is 5.8.
Coco can be used as the primary media to create a soil as well when including organic materials. This is the basis for Roots Organic soil. In that case you should be feeding at more along the lines of 6.5 to 7.0.
With Hydro pH levels being between 5.5 and 6.0 with 5.8 being ideal, and soil being 6.2 to 7.0 with 6.5 being ideal... you're not really in the right range for either method. This is likely making elements unavailable for uptake.
In addition to the pH value your planter of 5 gallons of coco is tremendously large. Large enough to grow 6 foot plus plants. The coco retention of moisture at that volume of coco is likely far too high and you may simply have too much media, too moist, for too long. I use 3 gallon smartpots and find them to even be too large. So I only use 2 gallons of space in the 3 gallon, this creates a wide base that demands frequent watering. Coco, as a hydroponic media, does not need to be in the same volume as soil for growing a similar sized plant.
I suggest you decrease the frequency of waterings and increase the volume of run off you are collecting. This can allow the coco to retain more air during the drier periods and stimulate root growth, but is only acceptable if you are watering at least once every 100 hours (every 4 days). Too infrequent and you will develop other issues.
Depending on the kind of "yellow" you have there are many different solutions to apply. Since I cannot be certain of any I suggest focusing on developing your root system as best you can to facilitate the growth of the plant and uptake of nutrients when in a tremendous amount of saturated media.
Also, when you water, dissolved oxygen in the solution replenishes the aeration value of the coco. The spongy physical structure of the coco allows it to retain a great deal of air but if you are going a very long time (more than 5 days) between watering because the coco just stays too wet the amount of air in the media will be reduced significantly. It is true that coco doesn't need to be cut with perlite, or another aeration media, because of this principle. What that depends on more is the frequency you are watering and the distance between these could result in drowning your root system which has not developed throughout the media.
Hence, with coco, size does matter. Ideally you don't want more media than you need. For most growers, an average 3 foot plant can be grown in just 2 gallons of coco, and the media will require frequent saturations. This helps keep the amount of air available to the root system high as it never becomes "stagnant."
Incorporate this knowledge and address your problems accordingly.