Their is ways to tell if your fish is doing well and one way is looking at their poop.
This chart is a general way of knowing how your fish is doing.
Long thin poop: Same color as there food but its thin and long. This normally caused by stress and not eating well. Fish should be closely watched.
White poop with air bubbles: There isnt any color to the poop and there is air trapped in the poop. This is signs of constipation, the food isnt being prossed in the fish. Fish should be not feed for three days and then feed nothing but peas for three days.
Thin white poop: There isnt any color to the poop. This is a sign of an infection inside of the fish and/or stress. Fish may need treatments. Look for other symptoms, and consult with an expert prior to starting any medication.
Long clear (zig zag) with air bubbles: There isnt any color to the poop but it is long and with air bubbles in the poop. This is normally the fish reabsorbing eggs.
Long and thick poop: The poop is long but the same color as the food you feed them but it trails the fish. This fish is fed to much and is a little constipated. Fish shouldnt be feed for three days.
Short to medium poop: Good healthy poop should be the same color as what you have feed them. The poop should be short to medium length and not be trailing the fish for long lengths of time.