Having read through this thread, I'll pick up on a couple of points.
First is the pH, and this is not an issue. One problem with our old plant profiles [we are moving the profiles to the new Reference Area, and I am revising them as I do] is the "Care Level" section near the top. This was devised to provide general information on light, temperature, etc. within the 3 categories of Easy, Moderate, Difficult (or whatever). Unfortunately, trying to give general data that fits a broad range of plants can cause more trouble than help, and this section turned out to be like this, so it is gone from the new profiles. If you look under "Water parameters" it says "acidic to slightly basic" without a pH number. You will not have a problem with most plants, including Cabomba, over pH.
On the liquid fertilizer, I agree with those who have recommended a broader, comprehensive product. Flourish Comprehensive Supplement is what I and many others here use, but another equal is Brightwell Aquatics' FlorinMulti. I think this was the one that Boredomb was trying to think of.
Either of these will help all plants, as they are complete and nutrients are in proportion according to aquatic plants' requirements.
On the temperature, 80F is high and while Cabomba might manage if this were the only issue, it also might not. Temperature has a considerable effect on plants, as on fish, the higher it goes. Temporary heat waves are normally not an issue, most of us face these in summer; but a continually high temperature does wear plants and fish out [when it is higher than their preference], and it might have been a contributor here.
That brings us to light, and this is the issue. I tried Cabomba in my 10g with two 10w CFL Daylight 6500K bulbs, and it too was fine for 5-6 weeks, then it began to fall apart until there were completely bare stems. This is a high-light plant. It is better to select another species that will manage. Finding one with the delicate ferny appearance of Cabomba is not easy as all of these require more light by their very nature. But as someone mentioned, plants often will manage in one tank but not another, even with very similar lighting, so you can experiment a bit and you might find one that works. Or you might not. One stem plant that does do well in moderate light is Brazilian Pennywort, though it looks nothing like Cabomba.
The issue of watts per gallon has been well covered, so I've nothing to add there.