Snap. I have that tank, with a betta.
Lighting can yield weak growth on a few stem plants, but it is hardly worth the bother.
Crypts, mosses and Monte carlo can make slow and weak growth. I know people that cram these tanks with plants and claim after just a few months that they work, but a long-term approach is more challenging.
NA Thrive and Flourish have advertised the correct nitrogen salts. Are their mixes validated or verified, I doubt. Only a few years ago we were eating horse meat advertised as beef, and we had mad cow disease due to livestock cannibalism - the producers got away with short cuts. I don't know who mixed your fertilizers, and personally I prefer to mix my own. But a more pragmatic approach would be far lower dosing; because without much light there is little point in going all out.
ANSWER = Gasping is usually a sign that oxygen has suddenly depleted. However, hemoglobin combines with some anions, effecting the hemoglobin-oxygen equilibrium. This is probably not due to an ionic strength effect but to the combination of anions with hemoglobin. The relationships are so complex that it is not possible to deduce one cause. But if I had to bet, I would say this occurred due to the fertilizer (as opposed to toxic shock, direct oxygen depletion, or gill irritation).
I presume that you were doing weekly 50% water changes.
Algae control is done by shrimp and with a credit card. I will never use Excel unless I get into the hospital floor cleaning industry, or suddenly want to end it all with a death cocktail. Children and Excel do not mix.
A smart upgrade would be one of those small 95 gram CO2 systems. I was going to try it on my Flex but didn't fancy any more work.