Hmm, I read your reply twice and I still have no idea how to test the tank for this. I'll look into it. I don't disagree with your observation, but with only feeding them once per day I guess I'm not sure how I could have done that. I did give them a decent amount of food, because I figured they were young and active and needed it. Would multiple smaller feedings per day be better? I've kept fish for a while and never had an issue like this. Also, geez... I thought Hikari was one of the best foods. There's such inconsistent guidance and information in this hobby! Why exactly don't you like Hikari foods?
People have different theories on feeding frequency. As far as I'm concerned there's not really a one-size-fits-all-fish answer, because what's better for adults vs young, growing juvies, larger predators vs grazers, etc. can vary. In any case, what's more important than feeding frequency is total feed intake and I control it by not allowing my fish to develop overgrown bellies. If I start to see it I back off feeding. Back in my Malawi cichlid days I'd even move an individual fish to another tank and put them on a 'diet' for a while, because they were too fast gobbling up food among their tankmates and getting a bulging belly. But it often seems to work feeding flakes or reducing pellet size to limit how fast any one fish can consume more than everyone else in the tank. I learned a long time ago that letting it persist or go too far can lead to issues.
HIkari has its fans and I'm not saying that's the main issue. Primarily, I don't like their ingredients, way too much cheap stuff in it. I figured out a long time ago there are better options in that respect, especially after doing some long term, side by side, same species, multi-tank testing... well, everyone has their opinions on food and this is just one more. But, for example, looking at Cichlid Gold, after fish meal the next 7 ingredients on their list are: wheat flour, flaked corn, brewers dried yeast, corn gluten meal, wheat starch, rice bran. So that's 7 to 1, with an awful lot of starchy, grain based, or similar items. Imo it's like feeding a fish flavored cracker. Compare to others like New Life Spectrum, Northfin, Fluval Bug Bites, etc.