Well, it is good to read that you observe your fishes poo, too.
So do I, like with little babies if you can see normal poo, all is fine and you do not worry.
So I do understand your concern very well.
After I fed Humpsi and Hepsi some peas, it happenes too, that I do not see a bit.
Then I feed crisps, they are very pink in colour and then it happens that I do see "healthy & thick" pink poopis and I am happy. Sometimes I can see those pink pieces on the white gravel, not for long though, because the filter sucks them away. Humphrey is a champ in producing those trails, Hepsiba may be a little shy and does it in the dark, lol
But the best thing is to observe, keep looking. Best time to do it when you are at home for one day.
I do have the "luxury" that our tank is a free standing one in the open plan living and kitchen area, so even if I cook I can see them and they can watch me too. ( Should have seen their eyes when I baked this orange-poppy seed - cake....) And when I comment their poo, my dear hubby replies: " Hannaaaa...." lol and I must laugh