I have had six danios in a 29 gallon tank for beginners since late March. Two separate fish have gotten similar looking objects lodged in their mouths at different times. One was able to dislodge it last month. I had the fish in a hospital tank and woke up every hour to check on it. I removed the object from the tank immediately after the fish dislodged it. I could not find anything else that looked like it in the tank. Three days ago, another had a similar-looking object in its mouth but kept trying to swallow it and died. I tried to lift him out with the net to help him dislodge it to no avail. I feed them TopFin tropical fish crumbles and frozen daphnia (after I thaw it) alternating. They have gravel in the tank, but it is differently colored and much larger than the objects. There are live plants, but I keep any flowering parts that are small trimmed off since the first incident. The objects are irregular in shape, hard, brown (slightly lighter than the lighting in the photo), and jagged. The pipette is for scale. It’s about the size of the danio’s mouth. Could the filter cartridge leak the tiny bits that are inside? I have run out of internet searches and ideas. Thank you.
My immediate thought, on seeing the photo, is that it's one of these https://en.wikipedia.org/wiki/Physidae which often arrive via live plants. If so, eggs may be being laid, and numbers thereby increasing. To stop numbers increasing, check for signs of eggs on the glass, around the tank, and especially behind the filter, and remove any that you see.
It is possible that pieces of filter cartridge (eg carbon) could dislodge which is a less likely explanation than the above; if it is this, you might need to consider transitioning to an alternative filter media or putting the foot of a pair of nylon tights over the internal filter output to prevent it disintegrating and ending up in the tank.
Thank you so much! I will inspect the walls and filter and probably remove each plant to check them before putting them back in the tank. I will also add a nylon to the filter until I verify the cause. I really appreciate your time and detailed explanation.