ok, that could very well be the problem, i like sand, but if it is a fine grain, then it could put a film into the water. when you stir it does it release a film? maybe stronger filtration. or a finer grained sand.
if a fish is continuisly pulled backwards, it can't move its gills in and out wich pump the oxygen through the body. so therefore if you would pull it backwards for longer than any fisherman would, then it would drown.
p.s. that is very cruel.